I am afraid, if the whole Maemo5 is designed to work with the black backgrounds only, then the only remaining option is to use a black background. Not doing it out of principle will do nothing to making your theme blend in with the actual applications.
Yea, BatteryGraph just looks a bit weird on the gray background, almost like there is something in the empty parts of the graph. Unfortunately, the Gallery, the Video Player, and anything that uses multicolumn lists are not blending in as well.