Avalonia/samples/MobileSandbox/App.xaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             Name="Mobile Sandbox"
             x:Class="MobileSandbox.App"
             RequestedThemeVariant="Dark">
  <Application.Styles>
    <FluentTheme />
  </Application.Styles>
</Application>