I'm a total noob for the car PC, and I too have been building skins as I think that's the only way to "know" what's going on behind the scenes. However, I also am struggling with a theme.
Some simple advice given to me (that you may not need) was to pick a consistent set of icons, use the same font throughout, etc. I certainly see many graphically beautiful skins here - almost all beyond my capability - but it is highly rewarding IMO to come up with your own stuff. Below is a pic of my main menu screen. I put the functions I call up most often there and built up other menu screens for less often used things (like DVD, image viewer, etc). I also went with space for big buttons as I found many to have buttons too small to for my use while driving. As a "theme" I made my buttons into stylized "keys" to fit with the car-puter theme. And finally, the central icon is the logo for my car (a Toyota MR2 Spyder), but I also use the space for other functions, i.e., the "M" at the top of the logo serves to minimize the screen, etc.
I have PSDs etc for all my screens, but no place to host them. You are more than welcome to rip off any idea I may help you get.