https://wccftech.com/facebook-messenger-dark-mode-is-now-available-for-everyone-toggle-it-straight-from-apps-settings/

What started off as a cool Easter Egg feature for Messenger, dark mode is now rolling out to all iOS and Android users around the globe.

Facebook Messenger managed to climb to the top of the charts and become the go-to messaging app for a lot of users around the world. To elevate the experience further, the social network giant brought dark mode to the masses which had to be enabled by sending a crescent emoji to another user. Essentially an Easter Egg in the app.