Facebook Messenger redesign began globally at the beginning of the year, dramatically changing the UI, making it more intuitive and easy to use. As part of the improvement, the company confirmed that the long-awaited dark mode will soon enter the app. While users can informally enable Dark Mode in the beta version of the app, it seems that recent updates to the stable version also allow users to enable dark mode in a rather unusual way, which is done through the emoji.
To enable Dark Mode, all you have to do is send a crescent moon emoji image emoji to any conversation in the chat list.
Doing so will trigger a smiling new moon emoji animation, for some reason, then you will need to manually close and reopen the app. After restarting the app, go to the settings page and see the mode switch at the top of the settings list.