https://thenextweb.com/facebook/2019/04/15/activate-facebook-messenger-dark-mode/

In March, Facebook teased Messenger’s long-awaited dark mode. Users could unlock the feature by simply sending a crescent moon emoji to one of their friends. And now, after a month of public experimentation, the feature is receiving a wide release and can be unlocked simply by activating a toggle.

To unleash dark mode, tap your profile photo within the app. This brings you to your preferences and settings. You’ll then see a switch marked “Dark Mode.” Press it and you’re done.