How to implement a multi-themed site? I will soon start building a new reasonably large site, that will be presented to the end-user as several different sites. Different domains will map to different sections within the site, and will also have different themes.

How should I implement the "multi-theme"?

I'll be posting some ideas of my own, which I would love some feedback on, but would also very much like to hear other suggestions.
I use the Themekey module.  This allows me to give each area a distinct look without even touching any code.  It also integrates with the Print, Revisioning, Organic Groups, and other major modules out-of-the-box.

Some of the things this module enables you to do (from the module page):


  
  individually-styled channels
  a front-page / "splash" screen
  a date/time-selected Christmas theme
  mobile themes for different auto-detected mobile devices
  special themes for "limited" or "old" browsers
  content, user, or role -specific themes
  indicating your environment (production, staging, testing, sandbox, … )
  testing your redesign safely on a live server