Hmm. Oh, you know, I've never actually tried to do it as a straight substitution. I've always done it through CSS, and you don't get the user over-riding them being connected that way.

thing-x { color: $*color-thing-y; }

Sorry for not thinking it through.

Um, I don't think you can temporarily define functions in themes? I know that you can create variables inside the print stylesheet functions, though. I don't know exactly how it's done, alas.