If like me you use Themes but are always annoyed that they don't show at design time. Well styleSheetTheme is the answer.
The styleSheetTheme actually gets applied in very early in the page cycle, even before the page control properties. That way you can still override the styleSheetTheme at page and control level by the Theme property.
You can set it a page level like this...
<%@ Page Language="C#" StylesheetTheme="Theme1" %>
or (if like me) you prefer to set things like this globally you can use the web.config
<system.web>
<pages styleSheetTheme="Theme1" />
<system.web>
One thing to note is that setting EnableTheming=false doesn't stop applying the styleSheetTheme.
I just like it as I get design time view of my pages.
Steve