RadControls for ASP.NET AJAX
Skins control the overall look and feel of a RadWindow object. By using Skins consistently, you can make your windows fit in with the design of the rest of a Web site, increasing usability and attractiveness. If you use the RadWindow versions of the Alert, Prompt, and Confirm dialogs, you can use the same skin with these dialogs and have them blend in more harmoniously with your Web site than the built-in browser dialogs do.
Specify the Skin to use by setting the Skin property of the RadWindowManager object. All windows that the RadWindowManager controls then inherit that Skin, including the Alert, Prompt, and Confirm dialogs. You can override this default Skin by setting the Skin property on an individual RadWindow object.
Since Q3 2011 all the built-in skins (except the Default one) have been relocated to the Telerik.Web.UI.Skins.dll assembly.
With Q1 2013 three new skins have been added for RadWindow(BlackMetroTouch, Glow and Silk).
There is a number of built-in skins for the RadWindow control: