No, you can only use the colors that the theme gives you. You can't customize it. I wasn't really paying attention to the color of the blades. I just love those Batman backgrounds. A co-worker has one with Joker holding a joker card over half his face, Batman holding a batarang over half his face, and Harvey Dent holding a 'Vote for Harvey Dent' button over half his face. I would have liked to have seen that one included.
Anyways, the easiest thing for these theme makers to do is just slap a background image in there and use the default blade and text colors.