There have been a few attempts at creating a theme manager program for Visualprefs, Birdie e.t.c. The most complete was probably AmiChameleon, the website can still be found on Archive.org:
http://web.archive.org/web/200502050...eon/index.html
The downloads no longer work, but I have put my copy of the main archive in the zone - one image is corrupt but it should still work.
If there is any interest I will dig out some of the themes for it and put them in the zone too...