Facebook introduced a dark theme for its Messenger application.
What is known
During the month night mode
A number of other applications, including YouTube andTwitter also offer a dark mode. It can help extend phone battery life, especially with OLED screens. Meanwhile, if you believe the rumors, the following major versions of Android and iOS should include system-wide dark mode settings.
Recall that Facebook Messenger can return back to the main application. Perhaps the new mode will be available there.