リソースディクショナリーで使うときは、下記を追加でsysが使えたのでメモ
xmlns:sys="clr-namespace:System;assembly=mscorlib"
Resourcesは参考サイトのコピペ、自分メモ用に拝借です
<!-- スクロールバーの幅 -->
<sys:Double x:Key="ScrollBarSize">12</sys:Double>
<!-- ボタンの長さ -->
<sys:Double x:Key="ScrollBarRepeatButtonSize">16</sys:Double>
<!-- スクロールバーのマージン -->
<sys:Double x:Key="ScrollBarMarginSize">5</sys:Double>
<!-- スクロールバーの色-->
<SolidColorBrush x:Key="ScrollBarColorBrush" Color="#66ffffff" />
<!-- ボーダーの色 -->
<SolidColorBrush x:Key="ScrollBarBorderBrush" Color="#66ffffff" />
<!-- トラック(レーン)の色 -->
<SolidColorBrush x:Key="ScrollBarTrackBrush" Color="#33ffffff" />
<!-- 三角の色 -->
<SolidColorBrush x:Key="ScrollBarHilightBrush" Color="#ccffffff" />
<!-- ボタンを押した時の色 -->
<SolidColorBrush x:Key="ScrollBarPressedBrush" Color="#99ffffff" />
<!-- 使用不可の色 -->
<SolidColorBrush x:Key="ScrollBarDisabledBrush" Color="#44ffffff" />
<!-- 終端の三角ボタンのスタイル -->
<Style x:Key="ScrollBarRepeatButtonStyle" TargetType="{x:Type RepeatButton}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RepeatButton}">
<Border x:Name="Border" Margin="0" CornerRadius="0" Background="{StaticResource ScrollBarColorBrush}" BorderBrush="{StaticResource ScrollBarBorderBrush}" BorderThickness="1">
<Path HorizontalAlignment="Center" VerticalAlignment="Center" Fill="{StaticResource ScrollBarHilightBrush}" Data="{Binding Content, RelativeSource={RelativeSource TemplatedParent}}" />
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsPressed" Value="true">
<Setter TargetName="Border" Property="Background" Value="{StaticResource ScrollBarPressedBrush}" />
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="{StaticResource ScrollBarDisabledBrush}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- トラック(レーン)のスタイル -->
<Style x:Key="ScrollBarTrackStyle" TargetType="{x:Type RepeatButton}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RepeatButton}">
<Border Background="Transparent"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- つまみのスタイル -->
<Style x:Key="ScrollBarThumbStyle" TargetType="{x:Type Thumb}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Thumb}">
<Border CornerRadius="0" Background="{StaticResource ScrollBarColorBrush}" BorderBrush="{StaticResource ScrollBarBorderBrush}" BorderThickness="1" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- カスタムスクロールバーのスタイル -->
<Style x:Key="CustomScrollBarStyle" TargetType="{x:Type ScrollBar}">
<Setter Property="OverridesDefaultStyle" Value="true"/>
<Style.Triggers>
<!-- 縦向きのスクロールバー -->
<Trigger Property="Orientation" Value="Vertical">
<Setter Property="Width" Value="{StaticResource ScrollBarSize}"/>
<Setter Property="Height" Value="Auto" />
<Setter Property="Margin">
<Setter.Value>
<Thickness Left="0" Top="{StaticResource ScrollBarMarginSize}" Right="{StaticResource ScrollBarMarginSize}" Bottom="{StaticResource ScrollBarMarginSize}" />
</Setter.Value>
</Setter>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate>
<Grid>
<Grid.RowDefinitions>
<RowDefinition MaxHeight="{StaticResource ScrollBarRepeatButtonSize}"/>
<RowDefinition/>
<RowDefinition MaxHeight="{StaticResource ScrollBarRepeatButtonSize}"/>
</Grid.RowDefinitions>
<Border Grid.RowSpan="3" CornerRadius="0" Background="{StaticResource ScrollBarTrackBrush}" />
<RepeatButton Grid.Row="0" Style="{StaticResource ScrollBarRepeatButtonStyle}" Height="{StaticResource ScrollBarRepeatButtonSize}" Command="ScrollBar.LineUpCommand" Content="M 0 4 L 8 4 L 4 0 Z" />
<Track x:Name="PART_Track" Grid.Row="1" IsDirectionReversed="true">
<Track.DecreaseRepeatButton>
<RepeatButton Style="{StaticResource ScrollBarTrackStyle}" Command="ScrollBar.PageUpCommand" />
</Track.DecreaseRepeatButton>
<Track.Thumb>
<Thumb Style="{StaticResource ScrollBarThumbStyle}" Margin="0,1,0,1"/>
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton Style="{StaticResource ScrollBarTrackStyle}" Command="ScrollBar.PageDownCommand" />
</Track.IncreaseRepeatButton>
</Track>
<RepeatButton Grid.Row="2" Style="{StaticResource ScrollBarRepeatButtonStyle}" Height="{StaticResource ScrollBarRepeatButtonSize}" Command="ScrollBar.LineDownCommand" Content="M 0 0 L 4 4 L 8 0 Z"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Trigger>
<!-- 横向きのスクロールバー -->
<Trigger Property="Orientation" Value="Horizontal">
<Setter Property="Width" Value="Auto"/>
<Setter Property="Height" Value="{StaticResource ScrollBarSize}" />
<Setter Property="Margin">
<Setter.Value>
<Thickness Left="{StaticResource ScrollBarMarginSize}" Top="0" Right="{StaticResource ScrollBarMarginSize}" Bottom="{StaticResource ScrollBarMarginSize}" />
</Setter.Value>
</Setter>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition MaxWidth="{StaticResource ScrollBarRepeatButtonSize}"/>
<ColumnDefinition/>
<ColumnDefinition MaxWidth="{StaticResource ScrollBarRepeatButtonSize}"/>
</Grid.ColumnDefinitions>
<Border Grid.ColumnSpan="3" CornerRadius="0" Background="{StaticResource ScrollBarTrackBrush}" />
<RepeatButton Grid.Column="0" Style="{StaticResource ScrollBarRepeatButtonStyle}" Width="{StaticResource ScrollBarRepeatButtonSize}" Command="ScrollBar.LineLeftCommand" Content="M 4 0 L 4 8 L 0 4 Z" />
<Track x:Name="PART_Track" Grid.Column="1" IsDirectionReversed="false">
<Track.DecreaseRepeatButton>
<RepeatButton Style="{StaticResource ScrollBarTrackStyle}" Command="ScrollBar.PageLeftCommand" />
</Track.DecreaseRepeatButton>
<Track.Thumb>
<Thumb Style="{StaticResource ScrollBarThumbStyle}" Margin="1,0,1,0"/>
</Track.Thumb>
<Track.IncreaseRepeatButton>
<RepeatButton Style="{StaticResource ScrollBarTrackStyle}" Command="ScrollBar.PageRightCommand" />
</Track.IncreaseRepeatButton>
</Track>
<RepeatButton Grid.Column="2" Style="{StaticResource ScrollBarRepeatButtonStyle}" Width="{StaticResource ScrollBarRepeatButtonSize}" Command="ScrollBar.LineRightCommand" Content="M 0 0 L 4 4 L 0 8 Z"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
</Style>
##参考サイト
XAML:StaticResourceとStyleの利用
https://qiita.com/code0327/items/e1eeaec9687fa5acef80
[WPF] WPF でスクロールバーのデザインをカスタマイズ
http://gootara.org/library/2016/06/wpf-scb.html