Themes are used to define visual styles for web pages. Themes are made up of a set of elements: skins, Cascading Style Sheets (CSS), images, and other resources. Themes can be applied at the application, page, or server control level. By storing these elements within the App_Themes Folder all the pages (including controls in page) in the solution, we take advantage of Theme. For more on Themes you can read my article "Working with Themes in ASP.NET 2.0."