ui/winui/App.xaml

<?xml version="1.0" encoding="utf-8"?>
<Application
    x:Class="winui.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:winui">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />
                <!-- Other merged dictionaries here -->
            </ResourceDictionary.MergedDictionaries>
            <!-- Other app resources here -->
            <SolidColorBrush x:Key="WindowCaptionBackground">Transparent</SolidColorBrush>
            <SolidColorBrush x:Key="WindowCaptionBackgroundDisabled">Transparent</SolidColorBrush>
        </ResourceDictionary>
    </Application.Resources>
</Application>