Applying a theme application-wide from the Web.config file
<?xml version="1.0" encoding="UTF-8" ?>
<configuration>
<system.web>
<pages theme="SmokeAndGlass" />
</system.web>
</configuration>
If you specify the theme in the Web.config file, you don't need to define the theme in the Page directive of your ASP.NET pages.
This theme is applied automatically to each and every page within your application.