mc:AlternateContent, mc:Choice

AlternateContent.cs

using System.Windows.Markup;

#if NET8_0
[assembly: XmlnsDefinition("http://schemas.custom.com/net8/xaml", "KsWare.Presentation.Themes.Core.Net8")]
#endif
namespace KsWare.Presentation.Themes.Core.Net8  {class Dummy{} }

AlternateContent.xaml

<Window 
	x:Class="Aeor2DarkTestApp.MainWindow"
	xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
	xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
	xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
	xmlns:net8="http://schemas.custom.com/net8/xaml"
	mc:Ignorable="d net8"
>
	<Grid>
		<mc:AlternateContent>
			<mc:Choice Requires="net8">
				<TextBlock Text="This is .NET 8 or higher!" 
						   HorizontalAlignment="Center" 
						   VerticalAlignment="Center" 
						   FontSize="20" 
						   Foreground="Green"/>
			</mc:Choice>
			<mc:Fallback>
				<TextBlock Text="This is below .NET 8!" 
						   HorizontalAlignment="Center" 
						   VerticalAlignment="Center" 
						   FontSize="20" 
						   Foreground="Red"/>
			</mc:Fallback>
		</mc:AlternateContent>
	</Grid>
</Window>