Look at the files in a skin folder.
Each screen has an '.skin' file has information like button position and name, plus a list of the four image files theat make up the screen. usually on, off, down and empty.
Make a copy of a skin (ie copy the folder) and have a play with the files inside, you should then get the idea.
BTW MediaCar is in a kind of dead situation, have a look at Road Runner, it is a copy of MediaCar but is actively being developed and hence has lots more functionality.