Doing such theming globally means that if a "custom" build is desired (say, for a different manufacturer who wanted their own custom look), the whole thing can be done centrally, in one location.

It also means that changing look and feel globally (as has happened with icons a couple of times before the current appearance) is a lot less painful, because you just swap out everything in one package and you're done - you don't need to land changes to a bunch of different places.