But all browsers can already do this. Most people don’t enable that setting, however. Could either be because they prefer to see the site as it’s intended to look, or because they’re unaware of the setting.
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.