Not just light vs. dark. I wish web sites would respect my system's preferences in general. If my OS theme is purple Comic Sans text on top of a yellow brick wall background, then my browser should respect that. I want to read text using the full width of the browser rather than a tiny 5 inch column down the middle of it, I shouldn't have to perform wizardry in the browser settings, conjure up some overriding CSS, or install extensions to do this. The browser should just say "tough shit, web developer, the user's preference wins."

Browsers have handed over way too much control to developers to ignore what the user wants. So much for being a "user agent." Browsers are more like the developer's agent.