Replying to myself to add more than a year later: enabling Dark Reader on restricted pages WITHOUT changing those settings contained in about:config mentioned above isn’t enough to get the extension working on addons.mozilla.org. In case anyone needed that clarification.
…Which I don’t love doing, even with juraj.masiar’s assurances. Even if the chances of something happening are very low, I’d rather now take the risk.
I just wish Firefox would get this setting up and going…and take less focus and money off the flipping AI.