The Dark Mode for Messenger is still in 'beta' mode, so it's not available for everyone . But there is a way to 'force' it on for your smartphone.
Step 1: Send the crescent moon emoji to a friend
Step 2: Done - Dark mode activated!
Once you send the emoji, your screen will 'rain' moons and a notification showing "You've found dark mode!" will appear:
Next, tap on "Try It in Settings" and toggle Dark mode 'on':
Congratulations! You have just enabled Dark mode on your smartphone's Messenger.
The screenshot on the left is the regular Messenger interface and the one on the right is with Dark mode enabled.
Pretty nifty, isn't it?