WPF-Demo/WpfSample/MainWindow.xaml.cs

using System.Windows;
using Waaagh.CustomControl;

namespace WpfSample {
    public partial class MainWindow: WaaaghWindow {
        ResourceDictionary _dark;
        ResourceDictionary _light;

        public MainWindow() {
            _dark = new() {
                Source = new Uri("/WaaaghTheme.Resources;component/DarkTheme.xaml", UriKind.Relative),
            };
            _light = new() {
                Source = new Uri("/WaaaghTheme.Resources;component/LightTheme.xaml", UriKind.Relative),
            };
            InitializeComponent();
        }

        private void DarkThemeButton_Click(object sender, RoutedEventArgs e) {
            Application.Current.Resources.MergedDictionaries.Remove(_dark);
            Application.Current.Resources.MergedDictionaries.Remove(_light);
            Application.Current.Resources.MergedDictionaries.Add(_dark);
        }

        private void LightThemeButton_Click(object sender, RoutedEventArgs e) {
            Application.Current.Resources.MergedDictionaries.Remove(_dark);
            Application.Current.Resources.MergedDictionaries.Remove(_light);
            Application.Current.Resources.MergedDictionaries.Add(_light);
        }
    }
}