You just got your answer go in the admin panel or work on the file itself. Have it backed up somewhere and then change colours according to the new theme you create. I usually make my layout in photoshop, slice and them code it in the forums (not in SMF since I wasn't able to do this unless I used another skin that was more "friendly" than the default one) .. I then pick colours from my own theme to make the CSS colours. I used find-replace in crimson editor (my editor for the codes) so that i can work faster. I always start from the layout in photoshop and end with the CSS, never the other way ...