You know, I was just thinking the same thing. I drive an '01 Viggen, and among the various restoration projects I have on my list of things to royallybotch up, I have replacing the radio with a carpc on there. It would certainly look a lot cooler if the interface was styled after OEM equipment (more so than the optional GM nav system currently available). I figure a low- or mid-res monocrhome theme in orange on black would look pretty cool. But why stop there? If you use the PC for diagnostics or gauges, make it look like the main gauge cluster. Either way, the high-contrast theme would also be great for driving at night, and the orange-on-black would also look good in a BMW. It would work well in any car that originally came with orange-on-black LCD displays or gauges. People put a lot of effort in making these things look awesome, when in a car you really need something simple and easy to read at a glance.
I don't think it would be at all difficult to do compared to other themes, given the proper tools. No gradients or anti-aliasing, just good old-fashioned 2-bit LCD goodness.