MainWindow.xaml.cs

using System;
using System.Windows;
using System.Windows.Input;

namespace CoreX
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            this.MouseLeftButtonDown += (s, e) =>
            {
                if (e.ButtonState == MouseButtonState.Pressed)
                    DragMove();
            };
        }

        private void SwitchTheme_Click(object sender, RoutedEventArgs e)
        {
            if (Application.Current.Resources.MergedDictionaries.Count == 0)
            {
                Application.Current.Resources.MergedDictionaries.Add(new ResourceDictionary()
                {
                    Source = new Uri("Themes/LightTheme.xaml", UriKind.Relative)
                });
            }

            var currentTheme = Application.Current.Resources.MergedDictionaries[0].Source.ToString();

            var newTheme = currentTheme.Contains("LightTheme")
                ? new Uri("Themes/DarkTheme.xaml", UriKind.Relative)
                : new Uri("Themes/LightTheme.xaml", UriKind.Relative);

            Application.Current.Resources.MergedDictionaries.Clear();
            Application.Current.Resources.MergedDictionaries.Add(new ResourceDictionary() { Source = newTheme });

            var newIconPath = currentTheme.Contains("LightTheme")
                ? "Assets/light.png"
                : "Assets/dark.png";

            ThemeIcon.Source = new System.Windows.Media.Imaging.BitmapImage(new Uri(newIconPath, UriKind.Relative));
        }

        private void Minimize_Click(object sender, RoutedEventArgs e)
        {
            this.WindowState = WindowState.Minimized;
        }

        private void CloseButton_Click(object sender, RoutedEventArgs e)
        {
            this.Close();
        }
    }
}