Added the black on white "accessible" theme -- this is meant for use on a monochrome desktop; white-on-black, blue/yellow and low contrast themes could be easyly done, if needed. But I was under the impression the plan was to recolor things on-the-fly with composite tricks or something like that.

It would be cool if somebody from the accessibility team could assess it's actually useful, or if changes are needed to make it better from the accessibility point of view.