Avalonia/samples/RenderDemo/App.xaml

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