@LeBiggenedQuack So... while this isn't a FandomDesktop thread: While FandomDesktop is not WHOLLY as convenient as I found Monobook before it was disabled, FandomDesktop is easier for my executive dysfunction brain to navigate than Oasis without custom CSS was. It's not perfect but it's far more accessible to me (so far). And in some ways it's appealing to me, too!
As for why Discussions is both not rebranded yet AND doesn't match FandomDesktop: It's not actually part of the wiki software--it just uses the same design language (or did). So while they probably have the goal of it matching, it's likely maintained separately and needs to be updated separately.