Avalonia/samples/AppWithoutLifetime/App.axaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="AppWithoutLifetime.App">
    <Application.Styles>
        <FluentTheme />
    </Application.Styles>
</Application>