If you create your custom skin under your custom theme folder (as it is in the sample Blue with right sidebar theme
), you don't need to reference anything in the SiteMenu.ascx user control. The skin will be applied automatically to the menu, when you select the needed theme from the dropdown listing the themes that are available for use by pages/templates in your Sitefinity-managed Web site.
I would recommend you read the following article which provides some general knowledge for the themes and skins in ASP.NET: ASP.NET Themes and Skins Overview
the Telerik team