The reason Apple doesn't support themes is so it has a total free hand with the look of OSX. Look at the Extras.rsrc for example, anyone who has even contemplated creating a Theme realises that this is a fairly complicated and essential file. With themeing unsupported Apple can do what it likes with this, it can add, move, erase and alter any of the elements in this file, it can do what it likes to change Aqua elements how it sees fit (e.g. the new Widgets for the 'textured' apps that appeared in 10.2.3 or the entire 'Textured app' option introduced with 10.2). If theming became Apple sanctioned this file would become 'Locked', with Apple unable to make any changes, no matter how small. This creates unflappability for Apple, they have nothing to gain in making Themes official.