It is part of the FB5 revamp
Coined as FB5, the latest update introduces a cleaner and simpler homepage header, a revamped user profile page, and the arrival of Dark Mode.
Here's a closer look at FB5, including Dark Mode for web, coming soon. pic.twitter.com/Z55iUOdekr
— Facebook (@Facebook) April 30, 2019
Currently available to developers and beta testers as of this writing, Facebook wants to add Dark mode to “help cut down on the glare and put photos and videos in focus.”
The current iteration of Dark Mode for mobile is limited to the Watch section, but expect the social media giant to expand Dark Mode to other parts of the app.