I've done a little customization of the interface. Added tabs, added a listing of open tickets to the main client welcome page, etc. If you search around the forums you will probably be able to find what you are looking for.
However, if you are looking to do more than just change a few little things here and there, or color changes you're going to have a rough time of it. I found that a lot of the page display parts are embedded in a lot of the different files.
i.e. this is no simple request.
Good luck!
