https://techcrunch.com/2020/03/19/how-to-use-facebook-dark-mode/

Facebook today officially rolled out its sweeping desktop redesign that offers dark mode, tabbed homescreen, and a cleaner profile. Facebook tells TechCrunch that “starting today, the majority of people on Facebook will have access to the new desktop design” which it announced at last year’s F8 conference. Users can opt-in to turn on the redesign before it becomes the default for everyone later this year.

To activate the Facebook.com redesign, go to the Settings Menu -> “See New Facebook”. To disable it, users can return and hit “Switch to Classic Facebook”.