On Monday Facebook confirmed the release of the hidden feature but also promised a full roll out in the "coming weeks."
Dark mode in Facebook Messenger can be enabled by sending a crescent moon emoji in Messenger. Facebook describes the process as "Simply send a crescent moon emoji – 🌙 – in any Messenger chat to unlock the setting and prompt to turn on dark mode." That said, many have found they may need to force quit Messenger or even reinstall it for the Dark mode to activate. There's been no word on a dark mode coming to Facebook proper.
Facebook also demonstrates that you can improve the look of your chats by tapping on the name, and selecting a custom color or gradient.