View Engine：定义在哪里查找具有后备视图的视图

Question

我在web.config中设置了一个主题，这意味着每个客户都可以为同一webshop加载不同的视图。

在Application_Start中，我有：

            RazorViewEngine viewEngine = new RazorViewEngine();

        var theme = System.Configuration.ConfigurationManager.AppSettings["Themes"];

        viewEngine.PartialViewLocationFormats =
            viewEngine.ViewLocationFormats =
                viewEngine.MasterLocationFormats =
                    new string[] {
                           "~/Assets/Views/{1}/{0}.cshtml",
                           "~/Assets/Views/Shared/{1}/{0}.cshtml"};
        viewEngine.AreaMasterLocationFormats =
           viewEngine.AreaPartialViewLocationFormats =
               viewEngine.AreaViewLocationFormats =
                   new string[] {
                           "~/Assets/Areas/{2}/Views/{1}/{0}.cshtml",
                           "~/Assets/Areas/{2}/Views/Shared/{1}/{0}.cshtml"};

        ViewEngines.Engines.Add(viewEngine);

这行得通，但是如果找不到视图文件，该如何定义后备？