https://www.theverge.com/2019/3/3/18248614/facebook-messenger-dark-mode-trick-to-activate-emoji

Facebook has been overhauling its Messenger app recently, slimming it down to focus more on chat, and promised that it would eventually roll out a dark mode for users. That mode is now rolling out, but there’s a tongue-in-cheek trick you need to do in order to activate it: you have to moon someone.

The trick, spotted by Android Police, 9to5Mac, and others, is simple: send someone (or yourself) a crescent moon emoji ( ). Once you do so, a shower of moons appears in the chat window, and you’ll get a prompt to activate the mode in settings. Go to your profile page in the app, and it’ll present you with an option to turn the mode on.