Window1.xaml.cs

using Wpf.Ui.Appearance;
using Wpf.Ui.Controls;

namespace TEST_WPF
{
    public partial class Window1 : FluentWindow
    {
        public Window1()
        {
            InitializeComponent();
        }

        private void FluentWindow_Loaded(object sender, System.Windows.RoutedEventArgs e)
        {
            ApplicationAccentColorManager.ApplySystemAccent();
            ApplicationThemeManager.ApplySystemTheme();
            Nav.Navigate("HomePage");
        }

        private void btnLightDarkSwitch_Click(object sender, System.Windows.RoutedEventArgs e)
        {
            ApplicationTheme currentAppTheme = ApplicationThemeManager.GetAppTheme();

            if (currentAppTheme == ApplicationTheme.Dark)
                ApplicationThemeManager.Apply(ApplicationTheme.Light, WindowBackdropType.None, updateAccent: true);
            else
                ApplicationThemeManager.Apply(ApplicationTheme.Dark, WindowBackdropType.None, updateAccent: true);
        }
    }
}