I have tried and spent many hours trying to figure out how to do this myself and have had no luck finding the correct key to call. If you are using winterboard it is something it will be able to do eventually but Saurik hasn't gotten around to it yet.
From: iPhone Theming on WinterBoard - Jay Freeman (saurik)
"I intend to support changing the style of a few other random elements (such as the Calendar icon and fake carrier strings), but I haven't had time to get around to that yet."