A Theme can contain one or more Skin files. A Skin modifies any of the appearance properties of an ASP.NET control. File: App_Themes\Simple\TextBox.skin BackColor="Yellow" BorderStyle="Dotted" Runat="Server" /> You can name a Skin file anything you want. A Theme folder can contain a single Skin file that contains Skins for all controls. A Theme folder can contain hundreds of Skin files, each of which contains a single Skin. Everything in a Theme folder gets compiled into one Theme class. You must include a Runat attribute, but you can never include the ID attribute when declaring a control in a Skin. File: ShowSkin.aspx uses the Simple Theme. Include a Theme attribute in its <%@ Page %> directive. <%@ Page Language="C#" Theme="Simple" %> "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> Show Skin