So why not just set the Theme or StyleSheetTheme in the @Page directive for
the page you want to appear differently?

If your not using a Theme or StyleSheetTheme you should look at the new 2.0
HtmlHead class that allows us to write into the head element noting we have
to mark the head runat="server" which breaks XHTML validation but so what?

// use google...
htmlhead site:msdn2.microsoft.com

Then you can simply use logic in the Page_Load event to determine which
stylesheet to load.

Now backtracking, if you are going to use Themes with MasterPages changing
the Theme dynamically requires doing so in Page_PreInit which the MasterPage
does not support. (OdeToCode.com has excellent articles about MasterPages).

When using Page_PreInit a base class that inherits from the Page class
becomes practical...

// a base class signature
public class MyBaseClass : System.Web.UI.Page

Note all other content pages must then inherit from your base class...

// content page signature
public class _Default.aspx : MyBaseClass

How's that?

<%= Clinton Gallagher
       NET csgallagher AT metromilwaukee.com
       URL http://www.metromilwaukee.com/clintongallagher/