ScrollBar.xaml

<Styles xmlns="https://github.com/avaloniaui">
    <Style Selector="ScrollBar">
        <Setter Property="Template">
            <ControlTemplate>
                <Border Background="{DynamicResource ThemeControlMidBrush}">
                    <Grid RowDefinitions="20,*,20">
                        <RepeatButton Name="PART_LineUpButton"
                                      Classes="repeat"
                                      Grid.Row="0"
                                      Grid.Column="0"
                                      Focusable="False">
                            <Path Data="M 0,4 C0,4 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 0,4 z"
                                  Stretch="Uniform"
                                  Fill="{DynamicResource ThemeForegroundLowBrush}" />
                        </RepeatButton>
                        <Track Grid.Row="1"
                               Grid.Column="1"
                               Minimum="{TemplateBinding Minimum}"
                               Maximum="{TemplateBinding Maximum}"
                               Value="{TemplateBinding Value, Mode=TwoWay}"
                               ViewportSize="{TemplateBinding ViewportSize}"
                               Orientation="{TemplateBinding Orientation}">
                            <Track.DecreaseButton>
                                <RepeatButton Name="PART_PageUpButton"
                                              Classes="repeattrack"
                                              Focusable="False"/>
                            </Track.DecreaseButton>
                            <Track.IncreaseButton>
                                <RepeatButton Name="PART_PageDownButton"
                                              Classes="repeattrack"
                                              Focusable="False"/>
                            </Track.IncreaseButton>
                            <Thumb Name="thumb">
                                <Thumb.Template>
                                    <ControlTemplate>
                                        <Border Background="{DynamicResource ThemeControlHighBrush}" />
                                    </ControlTemplate>
                                </Thumb.Template>
                            </Thumb>
                        </Track>
                        <RepeatButton Name="PART_LineDownButton"
                                      Classes="repeat"
                                      Grid.Row="2"
                                      Grid.Column="2"
                                      Focusable="False">
                            <Path Data="M 0,2.5 C0,2.5 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 0,2.5 z"
                                  Stretch="Uniform"
                                  Fill="{DynamicResource ThemeForegroundLowBrush}" />
                        </RepeatButton>
                    </Grid>
                </Border>
            </ControlTemplate>
        </Setter>
    </Style>
    <Style Selector="ScrollBar:horizontal">
        <Setter Property="Height"
                Value="20" />
        <Setter Property="Template">
            <ControlTemplate>
                <Border Background="{DynamicResource ThemeControlMidBrush}">
                    <Grid ColumnDefinitions="20,*,20">
                        <RepeatButton Name="PART_LineUpButton"
                                      Classes="repeat"
                                      Grid.Row="0"
                                      Grid.Column="0"
                                      Focusable="False">
                            <Path Data="M 3.18,7 C3.18,7 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 3.18,7 z"
                                  Stretch="Uniform"
                                  Fill="{DynamicResource ThemeForegroundLowBrush}" />
                        </RepeatButton>
                        <Track Grid.Row="1"
                               Grid.Column="1"
                               Minimum="{TemplateBinding Minimum}"
                               Maximum="{TemplateBinding Maximum}"
                               Value="{TemplateBinding Value, Mode=TwoWay}"
                               ViewportSize="{TemplateBinding ViewportSize}"
                               Orientation="{TemplateBinding Orientation}">
                            <Track.DecreaseButton>
                                <RepeatButton Name="PART_PageUpButton"
                                              Classes="repeattrack"
                                              Focusable="False"/>
                            </Track.DecreaseButton>
                            <Track.IncreaseButton>
                                <RepeatButton Name="PART_PageDownButton"
                                              Classes="repeattrack"
                                              Focusable="False"/>
                            </Track.IncreaseButton>
                            <Thumb Name="thumb">
                                <Thumb.Template>
                                    <ControlTemplate>
                                        <Border Background="{DynamicResource ThemeControlHighBrush}" />
                                    </ControlTemplate>
                                </Thumb.Template>
                            </Thumb>
                        </Track>
                        <RepeatButton Name="PART_LineDownButton"
                                      Classes="repeat"
                                      Grid.Row="2"
                                      Grid.Column="2"
                                      Focusable="False">
                            <Path Data="M 1.81,7 C1.81,7 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 1.81,7 z"
                                  Stretch="Uniform"
                                  Fill="{DynamicResource ThemeForegroundLowBrush}" />
                        </RepeatButton>
                    </Grid>
                </Border>
            </ControlTemplate>
        </Setter>
    </Style>
    <Style Selector="ScrollBar:horizontal /template/ Thumb#thumb">
        <Setter Property="MinWidth"
                Value="20" />
    </Style>
    <Style Selector="ScrollBar:vertical">
        <Setter Property="Width"
                Value="20" />
    </Style>
    <Style Selector="ScrollBar:vertical /template/ Thumb#thumb">
        <Setter Property="MinHeight"
                Value="20" />
    </Style>
    <Style Selector="ScrollBar /template/ RepeatButton.repeat">
        <Setter Property="Padding"
                Value="2" />
        <Setter Property="BorderThickness"
                Value="0" />
    </Style>
    <Style Selector="ScrollBar /template/ RepeatButton.repeattrack">
        <Setter Property="Template">
            <ControlTemplate>
                <Border Background="{TemplateBinding Background}" />
            </ControlTemplate>
        </Setter>
    </Style>
</Styles>