Many sites do not offer dark mode since it's a lot of work to maintain both bright and dark and some companies/site owners frankly do not care. For some people, dark mode is really important because bright pages cause eye strain. Especially if all the other application and operating system is themed in dark mode.
There are extensions already that can force dark mode but usually, its based on some algorithm that guesses the right colors correctly from time to time.
This proposal is about using AI to analyze sites and figure out a good way to turn styles and assets into a dark mode in the best possible way. It can be a switch in the menu but also a setting that can be saved per site and or domain/URL pattern.