ASP.NET themes are a collection of properties that enables to define the appearance of Web pages and controls on the website.
A theme can include skin files, cascading style sheet files (.css files), and graphics. By applying a theme, one can give Web pages a consistent appearance across the website.
Creating a Theme
A theme is used in Web applications by creating a skin file and attaching it to the Web pages on which the theme has to be applied. Skin files are used to define the property settings for ASP.NET Web server controls. Skin files are created and stored in a theme folder. This theme folder is placed inside the folder named App_Themes. All the themes in a Web application are placed inside the App_Themes folder. This folder is placed inside the top-level directory of the Web application.