I've got a Sansa E200 with rockbox r14391-070818 and have seen some issues with themes that contain a backdrop.
After you load a theme with a backdrop, and then load a theme without a backdrop, the backdrop stays.
I understand that you can select your own backdrops to be used independently of a theme but the problem arises when you have a theme with a black backdrop and the next theme has a black foreground font. You end up with a black screen.
This can be a scary thing for unsophisticated users whose next move would probably be to delete rockbox off their device.
I suggest that the guidelines state that all themes without a backdrop must be created with a blank backdrop: line in the cfg file. If you change themes and still want a backdrop, you can always re-select your backdrop and an unsuspecting user won't be left with a black screen.