I have always used the black but when some people post in certain colors i must highlight to read them- last time I saw the place where you can change colors it was at the bottom.
Well if his browser is now recognizing the new lack of theme options, then it would seem the cache has been cleared and I don't think e will be able to get the white back unless Avery adds it back in. One thing you could do is look for an addon which will reverse black and white. I know there is a Mac command for it, and there is probably a PC addon for Firefox which does the same thing....