You can enable Dark Mode in Facebook Messenger by sending crescent moon emoji
To get the Dark Mode, you need to send the yellow crescent moon emoji, exit the app, re-open it and then check ‘Settings’. The Dark Mode toggle will be the top option.
Send a 🌙 moon emoji to someone and then check your settings.
It usually works when a lot of moons start dropping / falling in the screen or making the emoji bigger by holding it down before sending.