I've put a couple of threads up in the past you might want to search for "gelaskins" for one but for me its limited to changing my Desktop picture and customising the Dock … then auto hiding it

If you can be bothered to customise the icons - I can't too much hassle for something so small - then CandyBar from Panic is one way of doing it.
I reckon if you have time to customise icons etc you don't have enough apps to keep you occupied
