I found this article for dark mode in Android https://support.google.com/chrome/answer/9275525?co=GENIE.Platform%3DAndroid&hl=en
That article only works for Chrome on Android though, and if there is anything similar for iOS I don't know, but as I know Apple was pretty full of bravura about a Dark Mode for Mac Desktop computers, I can hardly imagine they wouldn't add such a thing on iOS, but that's speculating.