Chrome(ium)-based browsers do have a dark mode which can be enabled in chrome:///flags, sure, and it works generally well but the fatal flaw is that it uses this ugly light gray color for the background instead of black(#000000) which looks 10x better. The only way to get a black background on webpages is using combinations of extensions like “Super Dark Mode” and “Dark Reader” which have an inherent flaw in that they cannot change the ugly gray color hardcoded in chrome that flashes in between page loads. So really the only way to do it would be to edit that gray color in the chromium source code and use a custom build, which is out of reach for most people for obvious reasons.