It’s not very pretty and in all honesty I can’t remember where it came from, there is something similar to what you’re after in one of my project here, there is probably a lot in the style that’s not required or is incorrect but may as well give it a shot.
<!--Scrollbar Thumbs-->
<Style x:Key="ScrollThumbs" TargetType="{x:Type Thumb}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Thumb}">
<Grid x:Name="Grid">
<Rectangle HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Width="Auto" Height="Auto" Fill="Transparent" />
<Border x:Name="Rectangle1" CornerRadius="10" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Width="Auto" Height="Auto" Background="{TemplateBinding Background}" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="Tag" Value="Horizontal">
<Setter TargetName="Rectangle1" Property="Width" Value="Auto" />
<Setter TargetName="Rectangle1" Property="Height" Value="7" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!--ScrollBars-->
<Style x:Key="{x:Type ScrollBar}" TargetType="{x:Type ScrollBar}">
<Setter Property="Stylus.IsFlicksEnabled" Value="false" />
<Setter Property="Foreground" Value="LightGray" />
<Setter Property="Background" Value="DarkGray" />
<Setter Property="Width" Value="10" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ScrollBar}">
<Grid x:Name="GridRoot" Width="19" Background="{TemplateBinding Background}">
<Grid.RowDefinitions>
<RowDefinition Height="0.00001*" />
</Grid.RowDefinitions>
<Track x:Name="PART_Track" Grid.Row="0" IsDirectionReversed="true" Focusable="false">
<Track.Thumb>
<Thumb x:Name="Thumb" Background="{TemplateBinding Foreground}" Style="{DynamicResource ScrollThumbs}" />
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton x:Name="PageUp" Command="ScrollBar.PageDownCommand" Opacity="0" Focusable="false" />
</Track.IncreaseRepeatButton>
<Track.DecreaseRepeatButton>
<RepeatButton x:Name="PageDown" Command="ScrollBar.PageUpCommand" Opacity="0" Focusable="false" />
</Track.DecreaseRepeatButton>
</Track>
</Grid>
<ControlTemplate.Triggers>
<Trigger SourceName="Thumb" Property="IsMouseOver" Value="true">
<Setter Value="{DynamicResource ButtonSelectBrush}" TargetName="Thumb" Property="Background" />
</Trigger>
<Trigger SourceName="Thumb" Property="IsDragging" Value="true">
<Setter Value="{DynamicResource DarkBrush}" TargetName="Thumb" Property="Background" />
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter TargetName="Thumb" Property="Visibility" Value="Collapsed" />
</Trigger>
<Trigger Property="Orientation" Value="Horizontal">
<Setter TargetName="GridRoot" Property="LayoutTransform">
<Setter.Value>
<RotateTransform Angle="-90" />
</Setter.Value>
</Setter>
<Setter TargetName="PART_Track" Property="LayoutTransform">
<Setter.Value>
<RotateTransform Angle="-90" />
</Setter.Value>
</Setter>
<Setter Property="Width" Value="Auto" />
<Setter Property="Height" Value="12" />
<Setter TargetName="Thumb" Property="Tag" Value="Horizontal" />
<Setter TargetName="PageDown" Property="Command" Value="ScrollBar.PageLeftCommand" />
<Setter TargetName="PageUp" Property="Command" Value="ScrollBar.PageRightCommand" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>