Avalonia/samples/ControlCatalog/SideBar.xaml

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>
    <Design.PreviewWith>
        <Border Padding="20">
            <TabControl Classes="sidebar">
                <TabItem Header="Item1"/>
                <TabItem Header="Item2"/>
            </TabControl> 
        </Border>
    </Design.PreviewWith>
    <Style Selector="TabControl.sidebar">
        <Setter Property="TabStripPlacement" Value="Left"/>
        <Setter Property="Padding" Value="8 0 0 0"/>
        <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush}"/>
        <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:pointerover">
        <Setter Property="Opacity" Value="1"/>
    </Style>
    <Style Selector="TabControl.sidebar > TabItem:pointerover">
        <Setter Property="Background" Value="Transparent"/>
    </Style>
    <Style Selector="TabControl.sidebar > TabItem:selected">
        <Setter Property="Opacity" Value="1"/>
    </Style>
    <Style Selector="TabControl.sidebar > TabItem:selected">
        <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush2}"/>
    </Style>
</Styles>