Avalonia/samples/ControlCatalog/App.xaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.App">
  <Application.Styles>        
    <StyleInclude Source="avares://Avalonia.Controls.DataGrid/Themes/Default.xaml"/>
    <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>
    <StyleInclude Source="/SideBar.xaml"/>
  </Application.Styles>
</Application>