As an Android user, I'm really a fan of "Dark Mode" or simply "Night Mode" on any apps whenever it's possible to use. Like on Twitter, Telegram Messaging app, and Reddit. Besides how cool it looks, dark mode saves lives—battery lives. It uses lesser battery power of your smartphones which is a better way to keep your phone live longer.
Update: Some iPhones can also enable this feature on their Facebook messenger app using the same method, which they have to update the app in App store.
So, How you can have Dark Mode on Facebook Messenger with Android and IOS?
To allow the mode, first you have to update the app on its latest version on play store (version 18.104.22.168.119) then you have to send a moon emoji to someone, using messenger app specifically the half crescent one.
After that, the screen will be filled with falling crescent moon emojis, like when you sent a snowflake emoji or a heart one. And there's a prompt message on the top of the app saying "You Found Dark Mode" and a button to redirect to app settings. There, you will finds the dark mode toggle to enable or disable the dark mode feature of the app.
When you enable this feature, you’ll also get a message that it’s still a work in progress and might not be available everywhere in Messenger for the time being.
You can see the step by step instructions on the animated image (GIF) below with my device.
Also, you don't necessarily have to send the crescent moon emoji to specific friend you can do it, just like what I did, I just send the emoji to a random messenger's bot, a spammy one.