Thought: What about a box on the user settings page in which a user can specify their own custom CSS? It would only apply when logged-in.
That means those who want dark mode can get dark mode, and those who want whatever other zany random style changes they want can get that too. (Yeah you can use an extension, but an extension isn't very friendly if you use different browsers on different devices–e.g I use Chrome on my Mac and Safari on my iPhone.)
html { background-color: #fff; filter: invert(1); } body > center > table > tbody > tr:first-of-type { filter: invert(1); }
Yeah, you could make micro adjustments, but that adds almost zero bloat...