I very much doubt this is at all possible. Themes were, let's face it, never meant to exist. It is therefore unlikely that Apple will have provided any hidden hooks to facilitate the easy "switching" of these themes. The only way I can think of you being able to do this is reverse engineering the various bits of the System that do GUI rendering (AppKit, Window Manager, etc) and basically the entire CoreGraphicsServices API, so that you could somehow kludge it into re-reading the Extras.rsrc file,a dn then sending a redraw message to every window (if that is even possible, I don't know).