terminal/src/cascadia/LocalTests_TerminalApp/TestHostApp/UnitTestApp.xaml

<Application x:Class="TestHostApp.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="using:TestHostApp"
             RequestedTheme="Dark" />