This extension provides dark-theme for both facebook.com and messenger.com. The theme is customizable from the options page. Also, the user can inject custom CSS styles to these websites to improve the styling and make the theme more personalized. You can simply turn the dark-theme on and off from the toolbar button. Also, it is possible to set a schedule (e.g. night-time duration only) to automatically turn the extension on and off.
Note that this theme is only meant to improve the Facebook usage and has nothing to do with the Faebook.com. It is a free open-source project.