PowerToys/src/core/Microsoft.PowerToys.Settings.UI/App.xaml

<xaml:XamlApplication
    x:Class="SettingsUI.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:xaml="using:Microsoft.Toolkit.Win32.UI.XamlHost"
    xmlns:local="using:SettingsUI">
    <Application.Resources>
        <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />
    </Application.Resources>
</xaml:XamlApplication>