Hah. Who'd have known.

I'm working on implementing this. There's a PR here:

https://github.com/RetroPie/EmulationStation/pull/168

If you want to help test it out, you can get it on my repository at the RetroPie-Virtual-Systems-PR branch.

https://github.com/pjft/EmulationStation/tree/RetroPie-Virtual-Systems-PR

You'll need the Comic Book theme or the Steampunk theme for it to work, for now. Otherwise those entries will be unthemed for now. Carbon is in the process of being updated.