ayrt i don't know which add-ons are the preferred ones now but as an example, past add-ons that created make-shift "dark mode" often still ran into issues with site images because they were on a white square background instead of being transparent. so you'd still end up blinded by certain image-heavy pages.

converting site images and media to not do that was definitely part of the refactor and in fact it might be better now for dark mode add-ons because of that.

this is ignoring what was needed at the code level in order for things to play nicely with each other, including allowing for player's using non-rule breaking custom add-ons. fr is an old site by internet standards and untangling code can be and usually is a nightmare game of Yeti in my Spaghetti.

another answer is that one player's preferred dark mode may be headache inducing to another player who wants a dark-but-not-that-dark mode and i personally suspect fr wants to launch a few different site skin options at the same time as "dark mode" so as to cover all bases and to not just stick players with choosing between "match in a cave" dark mode and "second coming of jesus" light mode. especially because their playerbase throws a fit if they feel "left out" by an update. making multiple site skins in this instance would therefore take additional time.