There is a Theme system setting variable...but...

[webmasterpdx]

Note, I've suggested this somewhere else, but I'll repeat it here. I see the obvious advantage in supporting ARGB8888, as there are color algorithms that people might want to research using the prime to emulate different color models (I've done some work like this in the past, and had I had an HP PRIME it would have been handy).
That being said, I also see a lot of possibilities for lower bitfield counts. e.g. 16 bits or even 4 bits per pixel. I can see a lot of uses for sprites with only 16 colors. Imagine a space invaders game. The invaders could be represented by sprites and no more than 16 colors would be needed for sure. The space they would take up would be much smaller and blitting would be faster. Worth considering.

As a sidebar, something that would make our programs versatile enough to still work when you change the firmware would be to allow us to determine which version of the bitfields are being used. It might be as simple as checking the software revision. Just let us know the best way you would recommend that we determine this.
Thank you
-Donald