Hmm, I'll poke the core2 code for that - I didn't notice, but most of the stuff I was doing had no gradient.

That code actually ties into custom_control_strip_colors :) It just presents the options in a different way than core2 seems to suggest (I have no idea why, I was fixing up someone else's code there). Checking both boxes sets custom_control_strip_colors to 'on_gradient', while leaving the background gradient unchecked sets it to 'on_no_gradient'.