That's because it's buried (and has been increasingly so over the decades), it'll mess up some sites and won't do anything on a bunch of others because no-one a

That's because it's buried (and has been increasingly so over the decades), it'll mess up some sites and won't do anything on a bunch of others because no-one accounts for user-defined colors/margins/fonts anymore when designing for the web, and it has more options than most people want/need to deal with. A simple browser-provided theme dropdown containing a few nice options, directly in the main browser chrome, from a major browser (perhaps Firefox, back when it still counted as "major") could have changed things completely—see the pressure on sites to support "dark mode" now that that's a simple option to enable. It'd be nearly impossible to change now, but 10-15 years ago, maybe it could have been changed, if any of the very small number of entities steering the direction of the web had tried.