I never change the mode of a website. I have rarely done so and it seems to me an absurd cost for nothing. If the site is well designed in light colors it shouldn't be a problem. If the site is badly designed then it would be normal to offer such a possibility. An operating system is different: you are 24 hours looking at it, it is normal to have display preferences but a website? The average number of visits to sites I have worked on has never exceeded 3 minutes.
Frankly, most of what I see of my OS these days is the status bar and the window decoration, a terminal window and maybe a file manager. Websites is what I stare at most of my day, and most of those are the same websites.
