Firefox does have a global setting to override the System setting if you want system dark but webpages light, for instance. Most browsers also support per-page

Firefox does have a global setting to override the System setting if you want system dark but webpages light, for instance. Most browsers also support per-page overrides, but the only consistent place to find it is Dev Tools, which is a shame. I think browsers decided to invest in "Reader Mode" as a UX over more direct control of user styles and page styles, and I'm not always sure that is the correct choice, but I can understand how it seems the simpler "one-button" choice.