This is sorta how dark reader and such works. It turns out that implementing dark mode for most websites is more complicated than inverting all the css colors. For example, some gray on white text might have enough contrast to be easily read, but when inverted the text is hard to discern or nearly invisible. Images too, they might have a white background but not look good when inverted. Native support is better

Funny enough they do. Before Dark Reader on Firefox on Android I had a Chrome flag that did the same thing. But Dark Reader does a better job IMO.