Avalonia/samples/RenderDemo/SideBar.xaml

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Style Selector="TabControl.sidebar">
    <Setter Property="TabStripPlacement" Value="Left"/>
    <Setter Property="Padding" Value="8 0 0 0"/>
    <Setter Property="Background" Value="{DynamicResource SystemAccentColor}"/>
    <Setter Property="Template">
      <ControlTemplate>
        <Border
            Margin="{TemplateBinding Margin}"
            BorderBrush="{TemplateBinding BorderBrush}"
            BorderThickness="{TemplateBinding BorderThickness}">
          <DockPanel>
            <ScrollViewer
                Name="PART_ScrollViewer"
                HorizontalScrollBarVisibility="{TemplateBinding (ScrollViewer.HorizontalScrollBarVisibility)}"
                VerticalScrollBarVisibility="{TemplateBinding (ScrollViewer.VerticalScrollBarVisibility)}"
                Background="{TemplateBinding Background}">
              <ItemsPresenter
                  Name="PART_ItemsPresenter"
                  Items="{TemplateBinding Items}"
                  ItemsPanel="{TemplateBinding ItemsPanel}"
                  ItemTemplate="{TemplateBinding ItemTemplate}">
              </ItemsPresenter>
            </ScrollViewer>
            <ContentPresenter
                Name="PART_SelectedContentHost"
                Margin="{TemplateBinding Padding}"
                HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                Content="{TemplateBinding SelectedContent}"
                ContentTemplate="{TemplateBinding SelectedContentTemplate}">
            </ContentPresenter>
          </DockPanel>
        </Border>
      </ControlTemplate>
    </Setter>
  </Style>

  <Style Selector="TabControl.sidebar > TabItem">
    <Setter Property="BorderThickness" Value="0"/>
    <Setter Property="Foreground" Value="White"/>
    <Setter Property="FontSize" Value="14"/>
    <Setter Property="Margin" Value="0"/>
    <Setter Property="Padding" Value="16"/>
    <Setter Property="Opacity" Value="0.5"/>
    <Setter Property="Transitions">
      <Transitions>
        <DoubleTransition Property="Opacity" Duration="0:0:0.150"/>
      </Transitions>
    </Setter>
  </Style>
  <Style Selector="TabControl.sidebar > TabItem:selected /template/ Border#PART_SelectedPipe">
    <Setter Property="IsVisible" Value="False" />
  </Style>
  <Style Selector="TabControl.sidebar > TabItem:pointerover">
    <Setter Property="Opacity" Value="1"/>
  </Style>
  <Style Selector="TabControl.sidebar > TabItem:pointerover /template/ Border#PART_LayoutRoot">
    <Setter Property="Background" Value="{DynamicResource SystemAccentColorLight2}"/>
  </Style>
  <Style Selector="TabControl.sidebar > TabItem:selected">
    <Setter Property="Opacity" Value="1"/>
  </Style>
  <Style Selector="TabControl.sidebar > TabItem:selected /template/ Border#PART_LayoutRoot">
    <Setter Property="Background" Value="{DynamicResource SystemAccentColorLight1}"/>
  </Style>
</Styles>