Avalonia/samples/ControlCatalog/SideBar.paml

<Styles xmlns="https://github.com/perspex">
  <Style Selector="TabControl.sidebar">
    <Setter Property="Template">
      <ControlTemplate>
        <Grid ColumnDefinitions="Auto,*">
          <Border Width="190" Background="#1976D2">
            <TabStrip Classes="sidebar"
                      Items="{TemplateBinding Items}"
                      SelectedIndex="{TemplateBinding Path=SelectedIndex, Mode=TwoWay}">
              <TabStrip.ItemsPanel>
                <ItemsPanelTemplate>
                  <StackPanel Orientation="Vertical" Gap="4" Margin="0,10,0,0"/>
                </ItemsPanelTemplate>
              </TabStrip.ItemsPanel>
            </TabStrip>
          </Border>
          <ScrollViewer Grid.Column="1">
            <Carousel Items="{TemplateBinding Items}"
                      MemberSelector="Content"
                      SelectedIndex="{TemplateBinding Path=SelectedIndex}"
                      Transition="{TemplateBinding Transition}"
                      Margin="8,0,0,0"/>
          </ScrollViewer>
        </Grid>
      </ControlTemplate>
    </Setter>
  </Style>

  <Style Selector="TabStrip.sidebar TabItem">
    <Setter Property="Foreground" Value="White"/>
    <Setter Property="FontSize" Value="14"/>
    <Setter Property="Margin" Value="0"/>
    <Setter Property="Padding" Value="8"/>
  </Style>

  <Style Selector="TabStrip.sidebar TabItem:selected">
    <Setter Property="Background" Value="#20ffffff"/>
  </Style>
</Styles>