The usual dark mode that has been available for a while on the desktop and iOS versions of Facebook is finally ready for a public test, although there is no release date to talk about at this stage. Since Instagram and WhatsApp got this visual theme a long time ago, Facebook for Android will probably go dark for everyone as soon as possible.
https://www.notebookcheck.net/Dark-Mode-finally-coming-to-Facebook-on-Android-but-not-yet.500941.0.html