<telerik:RadWindow x:Class="Example.MainWindow" 
                xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
                xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation" 
                xmlns:helpers="clr-namespace:Telerik.Windows.Controls.Theming.Helpers;assembly=Telerik.Windows.Controls" 
                helpers:WindowEffectsHelper.BackdropMaterial="None" 
                Header="RadWindow"> 

<telerik:RadWindow  xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation" x:Class="Example.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:local="clr-namespace:WindowWindows11ThemeTest"
                    mc:Ignorable="d"
                    
                    Style="{DynamicResource RadWindowStyle}"
                    
                    Header="MainWindow" Height="450" Width="800">

private bool isDarkPalette;

private void RadButton_Click(object sender, RoutedEventArgs e)
{
    if (this.isDarkPalette)
    {
        Windows11Palette.LoadPreset(Windows11Palette.ColorVariation.System);
        this.isDarkPalette = false;
    }
    else
    {
        Windows11Palette.LoadPreset(Windows11Palette.ColorVariation.Dark);
        this.isDarkPalette = true;
    }

    this.MergeDictionaries();
}

private void MergeDictionaries()
{
    Application.Current.Resources.MergedDictionaries.Clear();
    Application.Current.Resources.MergedDictionaries.Add(new ResourceDictionary()
    {
        Source = new Uri("/Telerik.Windows.Themes.Windows11;component/Themes/System.Windows.xaml", UriKind.RelativeOrAbsolute)
    });
    Application.Current.Resources.MergedDictionaries.Add(new ResourceDictionary()
    {
        Source = new Uri("/Telerik.Windows.Themes.Windows11;component/Themes/Telerik.Windows.Controls.xaml", UriKind.RelativeOrAbsolute)
    });
    Application.Current.Resources.MergedDictionaries.Add(new ResourceDictionary()
    {
        Source = new Uri("/Telerik.Windows.Themes.Windows11;component/Themes/Telerik.Windows.Controls.Navigation.xaml", UriKind.RelativeOrAbsolute)
    });
}