Avalonia/samples/ControlCatalog/App.xaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.App">
  <TrayIcon.TrayIcons>
    <TrayIcons>
      <TrayIcon Icon="/Assets/test_icon.ico" ToolTipText="Avalonia Tray Icon ToolTip">
        <NativeMenu.Menu>
          <NativeMenu>
            <NativeMenuItem Header="About" />
          </NativeMenu>
        </NativeMenu.Menu>
      </TrayIcon>
    </TrayIcons>
  </TrayIcon.TrayIcons>
  <Application.Styles>
    <Style Selector="TextBlock.h1">
      <Setter Property="FontSize" Value="16" />
      <Setter Property="FontWeight" Value="Medium" />
    </Style>
    <Style Selector="TextBlock.h2">
      <Setter Property="FontSize" Value="14" />
    </Style>
    <Style Selector="TextBlock.h3">
      <Setter Property="FontSize" Value="12" />
    </Style>
    <Style Selector="Label.h1">
      <Setter Property="FontSize" Value="16" />
      <Setter Property="FontWeight" Value="Medium" />
    </Style>
    <Style Selector="Label.h2">
      <Setter Property="FontSize" Value="14" />
    </Style>
    <Style Selector="Label.h3">
      <Setter Property="FontSize" Value="12" />
    </Style>
    <StyleInclude Source="/SideBar.xaml"/>
  </Application.Styles>
</Application>