In Visual Web Developer you can define page themes which you can then apply to one or more pages in your application. You can also create themes at the machine level that can be used in multiple applications on the server.
Themes consist of several supporting files, including style sheets for page appearance, control skins to define the appearance of server controls, and any other supporting images or files that make up the theme. The content of a theme is the same whether the theme is defined as a page theme or as a global theme.
Themes can be applied by using either the Theme or StyleSheetTheme attribute of the @ Page directive, or by setting the pages Element (ASP.NET Settings Schema) element in the application configuration file. Visual Web Developer will only visually represent themes applied by using the StyleSheetTheme attribute.
A global theme:
A global theme applies to all of the Web sites on a server. The location in which you create a folder for global themes depends on whether you are running your Web site using Internet Information Services (IIS) or testing it using the ASP.NET Development Server.