Side note: if you offer a dark theme and have a toggle "automatically switch based on system theme", make that the default option. And don't bury a dark theme under settings... I'm nearly certain that everyone that uses a dark theme wants to use it everywhere by default. And since we're kinda talking privacy here, don't rely on cookies... Everything is light theme by default, including the OS and browser. If users are using dark theme then they very likely made a conscious decision to do that.

Stop treating dark theme users as second class citizens. I'm also looking at you Wikipedia... There's like 30 browser addons, you can change the one line of code to solve all that...