Hello darkness my old friend. I’m here to flick your switch again.
Short Answer
In the Messenger app, click your own profile photo, then flick the “Dark Mode” switch directly.
In the Facebook app or on the web, it is not currently available, but it seems to be in development.

Dark Mode Coming to all of Facebook
Dark Mode is a trend that is being rolled out from all directions after Apple included it in iOS. And it also seems to be on its way to Facebook proper.
The Express, a UK newspaper, refers to Android Police when saying:
According to Android Police, a number of Facebook users have already been asked to test-out the flashy new design. Those who decided to accept the offer would be presented with the option to choose between a light or dark mode when logging into the social network.
The Express
The Dark Mode didn’t show up in the first major anticipated update (the “flashy new design” mentioned in the quote) after it had been announced. But we’re holding out for the Mode to be here with the next one.
Dark Mode for all of Facebook must be quite an undertaking thinking of what a tangled mass of functions and user content that app is, but they also clearly have the work force to get it done quickly.
Time will show.