Hello Jonathan,

yes u  must

see there http://quickstarts.asp.net/QuickStartv20/aspnet/doc/themes/personalization.aspx

---
WBR,
Michael  Nemtsev [.NET/C# MVP] :: blog: http://spaces.live.com/laflour 

"The greatest danger for most of us is not that our aim is too high and we
miss it, but that it is too low and we reach it" (c) Michelangelo

JW> If I want each user to be able to specify their own themes, does
JW> that mean I must set the theme in an even handler (such as PreInit)
JW> for every single page?
JW>
JW> I suppose I could cache the user's theme in the Session object. But
JW> I wondered if there was any shortcut to having to set the theme in
JW> the PreInit event for every single page.
JW>
JW> Thanks.
JW>

You can set the theme in Application_PreRequestHandlerExecute in
global.asax:

private void Application_PreRequestHandlerExecute(Object sender, EventArgs
e)
{
 HttpContext CurrentContext = HttpContext.Current;
 Page p = ( CurrentContext.Handler as Page );
 if ( null != p )
 {
    p.Theme = "Default";
  }
 }
}