Comment by mlindner

I'm not sure on your browser, but I've used this for quite a long time, since long before dark mode was available on most websites: https://addons.mozilla.org/en-US/firefox/addon/darkreader/

It turns most websites into a dark mode equivalent (mostly) correctly. Some websites it just doesn't work for and you need to disable it, but it's otherwise great. It also auto-detects (most of the time) when a site supports dark mode and shuts itself off for that site.