How to implement a multi-themed site?
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