Avalonia/tests/Avalonia.DesignerSupport.TestApp/App.xaml

<Application
    x:Class="Avalonia.DesignerSupport.TestApp.App"
    xmlns="https://github.com/avaloniaui"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Application.Styles>
        <StyleInclude Source="avares://Avalonia.Themes.Simple/DefaultTheme.xaml" />
        <StyleInclude Source="avares://Avalonia.Themes.Simple/Accents/Base.xaml" />
        <StyleInclude Source="avares://Avalonia.Themes.Simple/Accents/BaseLight.xaml" />
    </Application.Styles>
</Application>