Facebook got something innovative for You guys – a messenger dark mode. It is designed for the OLED screens (which every Galaxy S series phone have). On OLED-type screens, black tone doesn’t affect battery consumption at all. Currently, it’s hidden from the settings, but there’s a nifty trick which can enable it on your phone.
Messenger will be rolling the Dark Mode out to everyone in the settings by the end of March 2019 — without having to perform any tricks.
The dark mode setting can be unlocked by simply sending a crescent moon emoji. 
If dark mode enabled, it will show as a true black OLED mode where your pixels are turned off, so you’ll save battery life on phones with OLED screens like the iPhone XR or Galaxy S10. Which is ideal when you’re messaging people in bed at night. It’s a feature that users have demanded some time. The theme is still being worked on, which means that some cosmetic bugs are expected. Still, it’s already very usable.
It appears that dark mode will arrive as part of a wider Facebook Messenger redesign. Lately, Facebook has been changing trends. This dark mode update is most likely part of it’s minimal UX/UI look they have published this year.
