I remembered after my initial questions that the preinit event does not exist for a masterpage or user control, so that approach wouldn't work. Also the query string is read only so we wouldn't have much luck there.
I was able to look at the methods the PageRouteHandler class currently has and through some adjustments manually set the theme. This assumes you are using an ASP.NET theme in the ~/App_Themes folder.
For example the following path :
contains the following sub folders :
per the theme design guidelines in Telerik documentation. The following code would force any page to have that 'cool' theme.
If you have an embedded theme in an assembly this code might require adjustment. We combine / minify all our images and CSS via build steps and don't include them in a separate resources assembly.
Hope this helps!
Code Snippet :
ThemeManagerRouteHandler : Telerik.Sitefinity.Web.PageRouteHandler
IHttpHandler GetHttpHandler(RequestContext requestContext)
var handler = (Page)