In WPF/Silverlight ScrollViewer and ScrollBar control doesn't have any property to change color of Thumb dynamically from XAML by setting property. We have to edit the Style of ScrollBar and set the Brush for it according to our requirement. Based on requirement of one of the user on MSDN Forum I decided to create a Custom ScrollViewer which has a property to set Brush for Thumb.

Public Class CustomScrollViewer
    Inherits ScrollViewer
  
    Shared Sub New()
        DefaultStyleKeyProperty.OverrideMetadata(GetType(CustomScrollViewer), New FrameworkPropertyMetadata(GetType(CustomScrollViewer)))
    End Sub
  
    Public Property ScrollBarThumbBrush As Brush
        Get
            Return GetValue(ScrollBarThumbBrushProperty)
        End Get
  
        Set(ByVal value As Brush)
            SetValue(ScrollBarThumbBrushProperty, value)
        End Set
    End Property
  
    Public Shared ReadOnly ScrollBarThumbBrushProperty As DependencyProperty = DependencyProperty.Register("ScrollBarThumbBrush", GetType(Brush), GetType(CustomScrollBar), New PropertyMetadata(Brushes.Gray))
  
End Class


In the above class, I have created a ScrollBarThumbBrush property which we can use to set Brush for Thumb in ScrollBar. And following in the XAML style for CustomScrollViewer control

<Style TargetType="my:CustomScrollViewer">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="my:CustomScrollViewer">
                <Grid x:Name="Grid" Background="{TemplateBinding Background}">
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="*"/>
                        <ColumnDefinition Width="Auto"/>
                    </Grid.ColumnDefinitions>
                    <Grid.RowDefinitions>
                        <RowDefinition Height="*"/>
                        <RowDefinition Height="Auto"/>
                    </Grid.RowDefinitions>
                    <Rectangle x:Name="Corner" Grid.Column="1" Fill="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" Grid.Row="1"/>
                    <ScrollContentPresenter x:Name="PART_ScrollContentPresenter" CanContentScroll="{TemplateBinding CanContentScroll}" CanHorizontallyScroll="False" CanVerticallyScroll="False" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Grid.Column="0" Margin="{TemplateBinding Padding}" Grid.Row="0"/>
                    <ScrollBar x:Name="PART_VerticalScrollBar"   AutomationProperties.AutomationId="VerticalScrollBar" Cursor="Arrow" Grid.Column="1" Maximum="{TemplateBinding ScrollableHeight}" Minimum="0" Grid.Row="0" Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}" Value="{Binding VerticalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}" ViewportSize="{TemplateBinding ViewportHeight}"/>
                    <ScrollBar x:Name="PART_HorizontalScrollBar" AutomationProperties.AutomationId="HorizontalScrollBar" Cursor="Arrow" Grid.Column="0" Maximum="{TemplateBinding ScrollableWidth}" Minimum="0" Orientation="Horizontal" Grid.Row="1" Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}" Value="{Binding HorizontalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}" ViewportSize="{TemplateBinding ViewportWidth}"/>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

Now to allow Thumb change color dynamically we need to change the Style of ScrollBar and read ScrollBarThumbBrush property. As you can see in above XAML Style CustomScrollViewer contains a ScrollBar control, so CustomScrollViewer is Ancestor of ScrollBar. So we are going to use following binding code to read ScrollBarThumbBrush property value in ScrollBar.

Fill="{Binding Path=ScrollBarThumbBrush, RelativeSource={RelativeSource AncestorType=my:CustomScrollViewer}}"

Below is the complete Style of ScrollBar control

<Style TargetType="ScrollBar">
    <Setter Property="Stylus.IsPressAndHoldEnabled" Value="False"/>
    <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
    <Setter Property="Background" Value="#FFF0F0F0"/>
    <Setter Property="BorderBrush" Value="#FFF0F0F0"/>
    <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
    <Setter Property="BorderThickness" Value="1,0"/>
    <Setter Property="Width" Value="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}"/>
    <Setter Property="MinWidth" Value="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="ScrollBar">
                <Grid x:Name="Bg" SnapsToDevicePixels="True">
                    <Grid.RowDefinitions>
                        <RowDefinition MaxHeight="{DynamicResource {x:Static SystemParameters.VerticalScrollBarButtonHeightKey}}"/>
                        <RowDefinition Height="1E-05*"/>
                        <RowDefinition MaxHeight="{DynamicResource {x:Static SystemParameters.VerticalScrollBarButtonHeightKey}}"/>
                    </Grid.RowDefinitions>
                    <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.Row="1"/>
                    <RepeatButton x:Name="PART_LineUpButton" Command="ScrollBar.LineUpCommand" IsEnabled="{TemplateBinding IsMouseOver}">
                        <RepeatButton.Style>
                            <Style TargetType="{x:Type RepeatButton}">
                                <Setter Property="FocusVisualStyle">
                                    <Setter.Value>
                                        <Style>
                                            <Setter Property="Control.Template">
                                                <Setter.Value>
                                                    <ControlTemplate>
                                                        <Rectangle Margin="2" SnapsToDevicePixels="True" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
                                                    </ControlTemplate>
                                                </Setter.Value>
                                            </Setter>
                                        </Style>
                                    </Setter.Value>
                                </Setter>
                                <Setter Property="BorderThickness" Value="1"/>
                                <Setter Property="HorizontalContentAlignment" Value="Center"/>
                                <Setter Property="VerticalContentAlignment" Value="Center"/>
                                <Setter Property="Padding" Value="1"/>
                                <Setter Property="Focusable" Value="False"/>
                                <Setter Property="IsTabStop" Value="False"/>
                                <Setter Property="Template">
                                    <Setter.Value>
                                        <ControlTemplate TargetType="{x:Type RepeatButton}">
                                            <Border x:Name="border" BorderBrush="#FFF0F0F0" BorderThickness="1" Background="#FFF0F0F0" SnapsToDevicePixels="True">
                                                <ContentPresenter x:Name="contentPresenter" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" ContentStringFormat="{TemplateBinding ContentStringFormat}" Focusable="False" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                                            </Border>
                                            <ControlTemplate.Triggers>
                                                <Trigger Property="IsMouseOver" Value="True">
                                                    <Setter Property="Background" TargetName="border" Value="#FFDADADA"/>
                                                    <Setter Property="BorderBrush" TargetName="border" Value="#FFDADADA"/>
                                                </Trigger>
                                                <Trigger Property="IsPressed" Value="True">
                                                    <Setter Property="Background" TargetName="border" Value="#FF606060"/>
                                                    <Setter Property="BorderBrush" TargetName="border" Value="#FF606060"/>
                                                </Trigger>
                                                <Trigger Property="IsEnabled" Value="False">
                                                    <Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
                                                    <Setter Property="Background" TargetName="border" Value="#FFF0F0F0"/>
                                                    <Setter Property="BorderBrush" TargetName="border" Value="#FFF0F0F0"/>
                                                </Trigger>
                                            </ControlTemplate.Triggers>
                                        </ControlTemplate>
                                    </Setter.Value>
                                </Setter>
                            </Style>
                        </RepeatButton.Style>
                        <Path x:Name="ArrowTop" Data="M0,4C0,4 0,6 0,6 0,6 3.5,2.5 3.5,2.5 3.5,2.5 7,6 7,6 7,6 7,4 7,4 7,4 3.5,0.5 3.5,0.5 3.5,0.5 0,4 0,4z" Fill="#FF606060" Margin="3,4,3,3" Stretch="Uniform"/>
                    </RepeatButton>
                    <Track x:Name="PART_Track" IsDirectionReversed="True" IsEnabled="{TemplateBinding IsMouseOver}" Grid.Row="1">
                        <Track.DecreaseRepeatButton>
                            <RepeatButton Command="ScrollBar.PageUpCommand">
                                <RepeatButton.Style>
                                    <Style TargetType="{x:Type RepeatButton}">
                                        <Setter Property="OverridesDefaultStyle" Value="True"/>
                                        <Setter Property="Background" Value="Transparent"/>
                                        <Setter Property="Focusable" Value="False"/>
                                        <Setter Property="IsTabStop" Value="False"/>
                                        <Setter Property="Template">
                                            <Setter.Value>
                                                <ControlTemplate TargetType="{x:Type RepeatButton}">
                                                    <Rectangle Fill="{TemplateBinding Background}" Height="{TemplateBinding Height}" Width="{TemplateBinding Width}"/>
                                                </ControlTemplate>
                                            </Setter.Value>
                                        </Setter>
                                    </Style>
                                </RepeatButton.Style>
                            </RepeatButton>
                        </Track.DecreaseRepeatButton>
                        <Track.IncreaseRepeatButton>
                            <RepeatButton Command="ScrollBar.PageDownCommand">
                                <RepeatButton.Style>
                                    <Style TargetType="{x:Type RepeatButton}">
                                        <Setter Property="OverridesDefaultStyle" Value="True"/>
                                        <Setter Property="Background" Value="Transparent"/>
                                        <Setter Property="Focusable" Value="False"/>
                                        <Setter Property="IsTabStop" Value="False"/>
                                        <Setter Property="Template">
                                            <Setter.Value>
                                                <ControlTemplate TargetType="{x:Type RepeatButton}">
                                                    <Rectangle Fill="{TemplateBinding Background}" Height="{TemplateBinding Height}" Width="{TemplateBinding Width}"/>
                                                </ControlTemplate>
                                            </Setter.Value>
                                        </Setter>
                                    </Style>
                                </RepeatButton.Style>
                            </RepeatButton>
                        </Track.IncreaseRepeatButton>
                        <Track.Thumb>
                            <Thumb>
                                <Thumb.Style>
                                    <Style TargetType="{x:Type Thumb}">
                                        <Setter Property="OverridesDefaultStyle" Value="True"/>
                                        <Setter Property="IsTabStop" Value="False"/>
                                        <Setter Property="Template">
                                            <Setter.Value>
                                                <ControlTemplate TargetType="{x:Type Thumb}">
                                                    <Rectangle x:Name="rectangle" Fill="{Binding Path=ScrollBarThumbBrush, RelativeSource={RelativeSource AncestorType=my:CustomScrollViewer}}" Height="{TemplateBinding Height}" SnapsToDevicePixels="True" Width="{TemplateBinding Width}"/>
                                                    <ControlTemplate.Triggers>
                                                        <Trigger Property="IsMouseOver" Value="True">
                                                            <Setter Property="Fill" TargetName="rectangle" Value="#FFA6A6A6"/>
                                                        </Trigger>
                                                        <Trigger Property="IsDragging" Value="True">
                                                            <Setter Property="Fill" TargetName="rectangle" Value="#FF606060"/>
                                                        </Trigger>
                                                    </ControlTemplate.Triggers>
                                                </ControlTemplate>
                                            </Setter.Value>
                                        </Setter>
                                    </Style>
                                </Thumb.Style>
                            </Thumb>
                        </Track.Thumb>
                    </Track>
                    <RepeatButton x:Name="PART_LineDownButton" Command="ScrollBar.LineDownCommand" IsEnabled="{TemplateBinding IsMouseOver}" Grid.Row="2">
                        <RepeatButton.Style>
                            <Style TargetType="{x:Type RepeatButton}">
                                <Setter Property="FocusVisualStyle">
                                    <Setter.Value>
                                        <Style>
                                            <Setter Property="Control.Template">
                                                <Setter.Value>
                                                    <ControlTemplate>
                                                        <Rectangle Margin="2" SnapsToDevicePixels="True" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
                                                    </ControlTemplate>
                                                </Setter.Value>
                                            </Setter>
                                        </Style>
                                    </Setter.Value>
                                </Setter>
                                <Setter Property="BorderThickness" Value="1"/>
                                <Setter Property="HorizontalContentAlignment" Value="Center"/>
                                <Setter Property="VerticalContentAlignment" Value="Center"/>
                                <Setter Property="Padding" Value="1"/>
                                <Setter Property="Focusable" Value="False"/>
                                <Setter Property="IsTabStop" Value="False"/>
                                <Setter Property="Template">
                                    <Setter.Value>
                                        <ControlTemplate TargetType="{x:Type RepeatButton}">
                                            <Border x:Name="border" BorderBrush="#FFF0F0F0" BorderThickness="1" Background="#FFF0F0F0" SnapsToDevicePixels="True">
                                                <ContentPresenter x:Name="contentPresenter" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" ContentStringFormat="{TemplateBinding ContentStringFormat}" Focusable="False" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                                            </Border>
                                            <ControlTemplate.Triggers>
                                                <Trigger Property="IsMouseOver" Value="True">
                                                    <Setter Property="Background" TargetName="border" Value="#FFDADADA"/>
                                                    <Setter Property="BorderBrush" TargetName="border" Value="#FFDADADA"/>
                                                </Trigger>
                                                <Trigger Property="IsPressed" Value="True">
                                                    <Setter Property="Background" TargetName="border" Value="#FF606060"/>
                                                    <Setter Property="BorderBrush" TargetName="border" Value="#FF606060"/>
                                                </Trigger>
                                                <Trigger Property="IsEnabled" Value="False">
                                                    <Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
                                                    <Setter Property="Background" TargetName="border" Value="#FFF0F0F0"/>
                                                    <Setter Property="BorderBrush" TargetName="border" Value="#FFF0F0F0"/>
                                                </Trigger>
                                            </ControlTemplate.Triggers>
                                        </ControlTemplate>
                                    </Setter.Value>
                                </Setter>
                            </Style>
                        </RepeatButton.Style>
                        <Path x:Name="ArrowBottom" Data="M0,2.5C0,2.5 0,0.5 0,0.5 0,0.5 3.5,4 3.5,4 3.5,4 7,0.5 7,0.5 7,0.5 7,2.5 7,2.5 7,2.5 3.5,6 3.5,6 3.5,6 0,2.5 0,2.5z" Fill="#FF606060" Margin="3,4,3,3" Stretch="Uniform"/>
                    </RepeatButton>
                </Grid>
                <ControlTemplate.Triggers>
                    <MultiDataTrigger>
                        <MultiDataTrigger.Conditions>
                            <Condition Binding="{Binding IsMouseOver, ElementName=PART_LineDownButton}" Value="true"/>
                            <Condition Binding="{Binding IsPressed, ElementName=PART_LineDownButton}" Value="true"/>
                        </MultiDataTrigger.Conditions>
                        <Setter Property="Fill" TargetName="ArrowBottom" Value="White"/>
                    </MultiDataTrigger>
                    <MultiDataTrigger>
                        <MultiDataTrigger.Conditions>
                            <Condition Binding="{Binding IsMouseOver, ElementName=PART_LineUpButton}" Value="true"/>
                            <Condition Binding="{Binding IsPressed, ElementName=PART_LineUpButton}" Value="true"/>
                        </MultiDataTrigger.Conditions>
                        <Setter Property="Fill" TargetName="ArrowTop" Value="White"/>
                    </MultiDataTrigger>
                    <MultiDataTrigger>
                        <MultiDataTrigger.Conditions>
                            <Condition Binding="{Binding IsMouseOver, ElementName=PART_LineDownButton}" Value="true"/>
                            <Condition Binding="{Binding IsPressed, ElementName=PART_LineDownButton}" Value="false"/>
                        </MultiDataTrigger.Conditions>
                        <Setter Property="Fill" TargetName="ArrowBottom" Value="Black"/>
                    </MultiDataTrigger>
                    <MultiDataTrigger>
                        <MultiDataTrigger.Conditions>
                            <Condition Binding="{Binding IsMouseOver, ElementName=PART_LineUpButton}" Value="true"/>
                            <Condition Binding="{Binding IsPressed, ElementName=PART_LineUpButton}" Value="false"/>
                        </MultiDataTrigger.Conditions>
                        <Setter Property="Fill" TargetName="ArrowTop" Value="Black"/>
                    </MultiDataTrigger>
                    <Trigger Property="IsEnabled" Value="False">
                        <Setter Property="Fill" TargetName="ArrowTop" Value="#FFBFBFBF"/>
                        <Setter Property="Fill" TargetName="ArrowBottom" Value="#FFBFBFBF"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Style.Triggers>
        <Trigger Property="Orientation" Value="Horizontal">
            <Setter Property="Width" Value="Auto"/>
            <Setter Property="MinWidth" Value="0"/>
            <Setter Property="Height" Value="{DynamicResource {x:Static SystemParameters.HorizontalScrollBarHeightKey}}"/>
            <Setter Property="MinHeight" Value="{DynamicResource {x:Static SystemParameters.HorizontalScrollBarHeightKey}}"/>
            <Setter Property="BorderThickness" Value="0,1"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type ScrollBar}">
                        <Grid x:Name="Bg" SnapsToDevicePixels="True">
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition MaxWidth="{DynamicResource {x:Static SystemParameters.HorizontalScrollBarButtonWidthKey}}"/>
                                <ColumnDefinition Width="1E-05*"/>
                                <ColumnDefinition MaxWidth="{DynamicResource {x:Static SystemParameters.HorizontalScrollBarButtonWidthKey}}"/>
                            </Grid.ColumnDefinitions>
                            <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.Column="1"/>
                            <RepeatButton x:Name="PART_LineLeftButton" Command="ScrollBar.LineLeftCommand" IsEnabled="{TemplateBinding IsMouseOver}">
                                <RepeatButton.Style>
                                    <Style TargetType="{x:Type RepeatButton}">
                                        <Setter Property="FocusVisualStyle">
                                            <Setter.Value>
                                                <Style>
                                                    <Setter Property="Control.Template">
                                                        <Setter.Value>
                                                            <ControlTemplate>
                                                                <Rectangle Margin="2" SnapsToDevicePixels="True" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
                                                            </ControlTemplate>
                                                        </Setter.Value>
                                                    </Setter>
                                                </Style>
                                            </Setter.Value>
                                        </Setter>
                                        <Setter Property="BorderThickness" Value="1"/>
                                        <Setter Property="HorizontalContentAlignment" Value="Center"/>
                                        <Setter Property="VerticalContentAlignment" Value="Center"/>
                                        <Setter Property="Padding" Value="1"/>
                                        <Setter Property="Focusable" Value="False"/>
                                        <Setter Property="IsTabStop" Value="False"/>
                                        <Setter Property="Template">
                                            <Setter.Value>
                                                <ControlTemplate TargetType="{x:Type RepeatButton}">
                                                    <Border x:Name="border" BorderBrush="#FFF0F0F0" BorderThickness="1" Background="#FFF0F0F0" SnapsToDevicePixels="True">
                                                        <ContentPresenter x:Name="contentPresenter" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" ContentStringFormat="{TemplateBinding ContentStringFormat}" Focusable="False" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                                                    </Border>
                                                    <ControlTemplate.Triggers>
                                                        <Trigger Property="IsMouseOver" Value="True">
                                                            <Setter Property="Background" TargetName="border" Value="#FFDADADA"/>
                                                            <Setter Property="BorderBrush" TargetName="border" Value="#FFDADADA"/>
                                                        </Trigger>
                                                        <Trigger Property="IsPressed" Value="True">
                                                            <Setter Property="Background" TargetName="border" Value="#FF606060"/>
                                                            <Setter Property="BorderBrush" TargetName="border" Value="#FF606060"/>
                                                        </Trigger>
                                                        <Trigger Property="IsEnabled" Value="False">
                                                            <Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
                                                            <Setter Property="Background" TargetName="border" Value="#FFF0F0F0"/>
                                                            <Setter Property="BorderBrush" TargetName="border" Value="#FFF0F0F0"/>
                                                        </Trigger>
                                                    </ControlTemplate.Triggers>
                                                </ControlTemplate>
                                            </Setter.Value>
                                        </Setter>
                                    </Style>
                                </RepeatButton.Style>
                                <Path x:Name="ArrowLeft" Data="M3.18,7C3.18,7 5,7 5,7 5,7 1.81,3.5 1.81,3.5 1.81,3.5 5,0 5,0 5,0 3.18,0 3.18,0 3.18,0 0,3.5 0,3.5 0,3.5 3.18,7 3.18,7z" Fill="#FF606060" Margin="3" Stretch="Uniform"/>
                            </RepeatButton>
                            <Track x:Name="PART_Track" Grid.Column="1" IsEnabled="{TemplateBinding IsMouseOver}">
                                <Track.DecreaseRepeatButton>
                                    <RepeatButton Command="ScrollBar.PageLeftCommand">
                                        <RepeatButton.Style>
                                            <Style TargetType="{x:Type RepeatButton}">
                                                <Setter Property="OverridesDefaultStyle" Value="True"/>
                                                <Setter Property="Background" Value="Transparent"/>
                                                <Setter Property="Focusable" Value="False"/>
                                                <Setter Property="IsTabStop" Value="False"/>
                                                <Setter Property="Template">
                                                    <Setter.Value>
                                                        <ControlTemplate TargetType="{x:Type RepeatButton}">
                                                            <Rectangle Fill="{TemplateBinding Background}" Height="{TemplateBinding Height}" Width="{TemplateBinding Width}"/>
                                                        </ControlTemplate>
                                                    </Setter.Value>
                                                </Setter>
                                            </Style>
                                        </RepeatButton.Style>
                                    </RepeatButton>
                                </Track.DecreaseRepeatButton>
                                <Track.IncreaseRepeatButton>
                                    <RepeatButton Command="ScrollBar.PageRightCommand">
                                        <RepeatButton.Style>
                                            <Style TargetType="{x:Type RepeatButton}">
                                                <Setter Property="OverridesDefaultStyle" Value="True"/>
                                                <Setter Property="Background" Value="Transparent"/>
                                                <Setter Property="Focusable" Value="False"/>
                                                <Setter Property="IsTabStop" Value="False"/>
                                                <Setter Property="Template">
                                                    <Setter.Value>
                                                        <ControlTemplate TargetType="{x:Type RepeatButton}">
                                                            <Rectangle Fill="{TemplateBinding Background}" Height="{TemplateBinding Height}" Width="{TemplateBinding Width}"/>
                                                        </ControlTemplate>
                                                    </Setter.Value>
                                                </Setter>
                                            </Style>
                                        </RepeatButton.Style>
                                    </RepeatButton>
                                </Track.IncreaseRepeatButton>
                                <Track.Thumb>
                                    <Thumb>
                                        <Thumb.Style>
                                            <Style TargetType="{x:Type Thumb}">
                                                <Setter Property="OverridesDefaultStyle" Value="True"/>
                                                <Setter Property="IsTabStop" Value="False"/>
                                                <Setter Property="Template">
                                                    <Setter.Value>
                                                        <ControlTemplate TargetType="{x:Type Thumb}">
                                                            <Rectangle x:Name="rectangle" Fill="{Binding Path=ScrollBarThumbBrush, RelativeSource={RelativeSource AncestorType=my:CustomScrollViewer}}" Height="{TemplateBinding Height}" SnapsToDevicePixels="True" Width="{TemplateBinding Width}"/>
                                                            <ControlTemplate.Triggers>
                                                                <Trigger Property="IsMouseOver" Value="True">
                                                                    <Setter Property="Fill" TargetName="rectangle" Value="#FFA6A6A6"/>
                                                                </Trigger>
                                                                <Trigger Property="IsDragging" Value="True">
                                                                    <Setter Property="Fill" TargetName="rectangle" Value="#FF606060"/>
                                                                </Trigger>
                                                            </ControlTemplate.Triggers>
                                                        </ControlTemplate>
                                                    </Setter.Value>
                                                </Setter>
                                            </Style>
                                        </Thumb.Style>
                                    </Thumb>
                                </Track.Thumb>
                            </Track>
                            <RepeatButton x:Name="PART_LineRightButton" Grid.Column="2" Command="ScrollBar.LineRightCommand" IsEnabled="{TemplateBinding IsMouseOver}">
                                <RepeatButton.Style>
                                    <Style TargetType="{x:Type RepeatButton}">
                                        <Setter Property="FocusVisualStyle">
                                            <Setter.Value>
                                                <Style>
                                                    <Setter Property="Control.Template">
                                                        <Setter.Value>
                                                            <ControlTemplate>
                                                                <Rectangle Margin="2" SnapsToDevicePixels="True" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
                                                            </ControlTemplate>
                                                        </Setter.Value>
                                                    </Setter>
                                                </Style>
                                            </Setter.Value>
                                        </Setter>
                                        <Setter Property="BorderThickness" Value="1"/>
                                        <Setter Property="HorizontalContentAlignment" Value="Center"/>
                                        <Setter Property="VerticalContentAlignment" Value="Center"/>
                                        <Setter Property="Padding" Value="1"/>
                                        <Setter Property="Focusable" Value="False"/>
                                        <Setter Property="IsTabStop" Value="False"/>
                                        <Setter Property="Template">
                                            <Setter.Value>
                                                <ControlTemplate TargetType="{x:Type RepeatButton}">
                                                    <Border x:Name="border" BorderBrush="#FFF0F0F0" BorderThickness="1" Background="#FFF0F0F0" SnapsToDevicePixels="True">
                                                        <ContentPresenter x:Name="contentPresenter" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" ContentStringFormat="{TemplateBinding ContentStringFormat}" Focusable="False" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                                                    </Border>
                                                    <ControlTemplate.Triggers>
                                                        <Trigger Property="IsMouseOver" Value="True">
                                                            <Setter Property="Background" TargetName="border" Value="#FFDADADA"/>
                                                            <Setter Property="BorderBrush" TargetName="border" Value="#FFDADADA"/>
                                                        </Trigger>
                                                        <Trigger Property="IsPressed" Value="True">
                                                            <Setter Property="Background" TargetName="border" Value="#FF606060"/>
                                                            <Setter Property="BorderBrush" TargetName="border" Value="#FF606060"/>
                                                        </Trigger>
                                                        <Trigger Property="IsEnabled" Value="False">
                                                            <Setter Property="Opacity" TargetName="contentPresenter" Value="0.56"/>
                                                            <Setter Property="Background" TargetName="border" Value="#FFF0F0F0"/>
                                                            <Setter Property="BorderBrush" TargetName="border" Value="#FFF0F0F0"/>
                                                        </Trigger>
                                                    </ControlTemplate.Triggers>
                                                </ControlTemplate>
                                            </Setter.Value>
                                        </Setter>
                                    </Style>
                                </RepeatButton.Style>
                                <Path x:Name="ArrowRight" Data="M1.81,7C1.81,7 0,7 0,7 0,7 3.18,3.5 3.18,3.5 3.18,3.5 0,0 0,0 0,0 1.81,0 1.81,0 1.81,0 5,3.5 5,3.5 5,3.5 1.81,7 1.81,7z" Fill="#FF606060" Margin="3" Stretch="Uniform"/>
                            </RepeatButton>
                        </Grid>
                        <ControlTemplate.Triggers>
                            <MultiDataTrigger>
                                <MultiDataTrigger.Conditions>
                                    <Condition Binding="{Binding IsMouseOver, ElementName=PART_LineRightButton}" Value="true"/>
                                    <Condition Binding="{Binding IsPressed, ElementName=PART_LineRightButton}" Value="true"/>
                                </MultiDataTrigger.Conditions>
                                <Setter Property="Fill" TargetName="ArrowRight" Value="White"/>
                            </MultiDataTrigger>
                            <MultiDataTrigger>
                                <MultiDataTrigger.Conditions>
                                    <Condition Binding="{Binding IsMouseOver, ElementName=PART_LineLeftButton}" Value="true"/>
                                    <Condition Binding="{Binding IsPressed, ElementName=PART_LineLeftButton}" Value="true"/>
                                </MultiDataTrigger.Conditions>
                                <Setter Property="Fill" TargetName="ArrowLeft" Value="White"/>
                            </MultiDataTrigger>
                            <MultiDataTrigger>
                                <MultiDataTrigger.Conditions>
                                    <Condition Binding="{Binding IsMouseOver, ElementName=PART_LineRightButton}" Value="true"/>
                                    <Condition Binding="{Binding IsPressed, ElementName=PART_LineRightButton}" Value="false"/>
                                </MultiDataTrigger.Conditions>
                                <Setter Property="Fill" TargetName="ArrowRight" Value="Black"/>
                            </MultiDataTrigger>
                            <MultiDataTrigger>
                                <MultiDataTrigger.Conditions>
                                    <Condition Binding="{Binding IsMouseOver, ElementName=PART_LineLeftButton}" Value="true"/>
                                    <Condition Binding="{Binding IsPressed, ElementName=PART_LineLeftButton}" Value="false"/>
                                </MultiDataTrigger.Conditions>
                                <Setter Property="Fill" TargetName="ArrowLeft" Value="Black"/>
                            </MultiDataTrigger>
                            <Trigger Property="IsEnabled" Value="False">
                                <Setter Property="Fill" TargetName="ArrowLeft" Value="#FFBFBFBF"/>
                                <Setter Property="Fill" TargetName="ArrowRight" Value="#FFBFBFBF"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Trigger>
    </Style.Triggers>
</Style>

Now we can use the CustomScrollViewer control and set ScrollBarThumbBrush property as shown in following code in the project where we need Thumb color as per our requirement.

<my:CustomScrollViewer ScrollBarThumbBrush="Blue"  HorizontalScrollBarVisibility="Visible" VerticalScrollBarVisibility="Visible">
     <Grid Width="600" Height="600"/>
</my:CustomScrollViewer>