This is How You Turn Your Facebook Messenger Into Dark Mode
Facebook has a secret update for your Messenger called Dark Mode and it's so much better than the original. The mode turns the background black. This feature has been out for a few months but hasn't been announced so that they could slowly roll it out.
To activate the Dark Mode on your messenger send the crescent moon emoji ( 🌙 this one, specifically) to anyone through messenger. Then exit out of the app and when you come back to click the moon, or watch for the message about the moon for you to click and take you to the settings to activate the Dark Mode... Once you activate it you'll see the black background during all your messages.
So far I only see the Dark Mode on my mobile app but I still love it!