I was wondering if it was possible to skin the AVIC D3's user interface. If it would be possible by modifying an update file, or another method instead of just simply changing the background.

Thanks

Same question here. In particular it would be great to change the ilumination  of the buttons and the soft keys to red (audi)

Anything is possible but finding someone to do it is the problem. Its highly doubtful on the 7 year old unit that most people no longer have.