ASP.NET Tutorial/Configuration/styleSheetTheme

Register a Theme for all pages in your application in the web configuration file.

File: Web.Config
<configuration>
<system.web>
  <pages theme="Site" />
</system.web>
</configuration>

Use the styleSheetTheme attribute to apply a Theme to the pages in an application

If you use the styleSheetTheme attribute, you can override particular Skin properties in a page.
File: Web.Config
<configuration>
<system.web>
  <pages styleSheetTheme="Site" />
</system.web>
</configuration>