Instead of arbitrarily clearing the backdrop, why don't we instead trust people to be able to decide for themselves what they'd like the backdrop to be?
You can easily change the backdrop for the main theme and pick one that doesn't interfere with the WRS, meanwhile arbitrarily clearing it for any theme is nearly as dangerous as some may not interfere at all while the actual backdrop color does, so the only solution other than "trust the users" is "pick a single foreground and background color combination and force it on all users". Just let users handle it themselves by picking a theme that doesn't interfere until such time as it may become customizable, if it does.