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

<xaml:XamlApplication
    x:Class="Microsoft.PowerToys.Settings.UI.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">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <XamlControlsResources  xmlns="using:Microsoft.UI.Xaml.Controls" />
                <ResourceDictionary Source="/Styles/_Colors.xaml" />
                <ResourceDictionary Source="/Styles/_FontSizes.xaml" />
                <ResourceDictionary Source="/Styles/_Thickness.xaml" />

                <ResourceDictionary Source="/Styles/TextBlock.xaml" />
                <ResourceDictionary Source="/Styles/Page.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</xaml:XamlApplication>