DirectoryCreator/ThemeManager.cs

using System.Windows;

namespace DirectoryCreator
{
    public static class ThemeManager
    {
        public static void SetTheme(bool isDarkTheme)
        {
            var app = Application.Current;
            var mergedDicts = app.Resources.MergedDictionaries;

            // Ruta al tema oscuro
            var darkThemeUri = new Uri("Themes/DarkTheme.xaml", UriKind.Relative);

            // Primero, removemos el tema oscuro si existe
            for (int i = mergedDicts.Count - 1; i >= 0; i--)
            {
                var currentDict = mergedDicts[i];
                if (currentDict.Source != null && currentDict.Source.Equals(darkThemeUri))
                {
                    mergedDicts.RemoveAt(i);
                }
            }

            // Si se solicita el tema oscuro, lo agregamos
            if (isDarkTheme)
            {
                var darkTheme = new ResourceDictionary
                {
                    Source = darkThemeUri
                };
                mergedDicts.Add(darkTheme);
            }
        }
    }
}