Вам придется изменить шаблон ScrollViewer.
<ControlTemplate TargetType="{x:Type ScrollViewer}">
<Grid Background="{TemplateBinding Background}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<ScrollBar
x:Name="PART_VerticalScrollBar"
Grid.Column="1"
Minimum="0.0"
Maximum="{TemplateBinding ScrollableHeight}"
ViewportSize="{TemplateBinding ViewportHeight}"
Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=VerticalOffset, Mode=OneWay}"
Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}"
Cursor="Arrow"
AutomationProperties.AutomationId="VerticalScrollBar"
Style="{StaticResource MyScrollBarStyle}"/>
<ScrollBar
x:Name="PART_HorizontalScrollBar"
Orientation="Horizontal"
Grid.Row="1"
Minimum="0.0"
Maximum="{TemplateBinding ScrollableWidth}"
ViewportSize="{TemplateBinding ViewportWidth}"
Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=HorizontalOffset, Mode=OneWay}"
Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}"
Cursor="Arrow"
AutomationProperties.AutomationId="HorizontalScrollBar"
Style="{StaticResource MyScrollBarStyle}"/>
<ScrollContentPresenter
x:Name="PART_ScrollContentPresenter"
Margin="{TemplateBinding Padding}"
Content="{TemplateBinding Content}"
ContentTemplate="{TemplateBinding ContentTemplate}"
CanContentScroll="{TemplateBinding CanContentScroll}"/>
<Rectangle
x:Name="Corner"
Grid.Column="1"
Grid.Row="1"
Fill="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
</Grid>
Или вы можете удалить ключ стиля, чтобы он применялся ко всем элементам управления ScrollBar. Надеюсь, это поможет.