First, I apologise to gwlxxx for possibly hijacking the thread, but I believe AmigaSYS offers some kind of multi-theme switching (through an interface). Furthermore, there was a website out there not so long ago that offered software in the form of batch scripts that achieved something approaching your description.
As for AIAB's different theme styles, you're right, it could be easier, but for the nostalgic amongst us, take a look at how the AIAB default interface has changed over the past few years. Note that all of the old styles are still included in the latest package.

In my opinion, the latest 'look' is the best, however I know some folks prefer the older XEN interface, whereas others prefer no UI modifications whatsoever!
Cheers, take it easy!
James