Default ASP.NET Themes

This functionality isn't built into ASP.NET. Nevertheless, you could implement it fairly easily:

Hook the HttpApplication.BeginRequest event in Global.asax or in a custom HTTP module. Look for requests with URLs under "/App_Themes/NewTheme/". Check whether the file at HttpRequest.PhysicalPath exists. If the file doesn't exist, call HttpContext.RewritePath and replace "NewTheme" in the request URL with "Default".

Answers

Default themes as you describe aren't supported by ASP.NET. There are regular Themes and StyleSheetThemes, but changing them dynamically is more useful at the Page request level than for individual Controls or static files.

You can code up your own version of themes for static files using URL rewriting or routing -- but then it's not really Themes any more.

For controls like <asp:Image>, you could override them and modify properties such as ImageUrl based on which files exist in some hierarchy of "theme" folders. Then use tag mapping to replace all instances of that control with the new one, without requiring any markup changes.

FWIW, the BeginRequest event in Global.asax is only invoked for dynamic files in IIS (Cassini calls it for statics, too). To support statics in IIS, you'll need an HttpModule, and you'll also need to configure IIS to run in Integrated mode.