It seems like more websites are going to dark mode. My iPhone now goes to dark mode at night, because of one of the Apple updates. I seldom even use my phone, so I have not bothered to go to settings and change it back to regular more all the time, but I did with the ipad because I use it at night a lot.
Mine is in settings under display and brightness. I have it set on automatic on the phone so it changes at night to the dark mode. I also have the blue-blocker turned on, so at night, it goes into a warmer light, which is better for being able to fall asleep.