Avalonia/samples/ControlCatalog/App.xaml

<Application xmlns="https://github.com/avaloniaui">
  <Application.Styles>
    <StyleInclude Source="resm:Avalonia.Themes.Default.DefaultTheme.xaml?assembly=Avalonia.Themes.Default"/>
    <StyleInclude Source="resm:Avalonia.Themes.Default.Accents.BaseLight.xaml?assembly=Avalonia.Themes.Default"/>
    <Style Selector="TextBlock.h1">
      <Setter Property="FontSize" Value="{DynamicResource FontSizeLarge}"/>
      <Setter Property="FontWeight" Value="Medium"/>
    </Style>
    <Style Selector="TextBlock.h2">
      <Setter Property="FontSize" Value="{DynamicResource FontSizeNormal}"/>
    </Style>
    <Style Selector="TextBlock.h3">
      <Setter Property="FontSize" Value="{DynamicResource FontSizeSmall}"/>
    </Style>
    <StyleInclude Source="resm:ControlCatalog.SideBar.xaml"/>
  </Application.Styles>
</Application>