public App()
{

    ResourceDictionary dict = new ResourceDictionary();
    dict.Source = new Uri("Themes/ExpressionDark.xaml",UriKind.Relative);
    Resources.MergedDictionaries.Add(dict);

    //this.Resources.MergedDictionaries.Add(Application.LoadComponent( new Uri("Themes/ExpressionDark.xaml", UriKind.Relative)) as ResourceDictionary);

}