This article explains how to enable dark mode on the Facebook website, the Facebook app for iOS and Android, Google Chrome, and other Chromium-based browsers.
How to Enable Facebook Dark Mode
Dark Mode on Facebook changes the white-and-blue color scheme to a dark gray background with white text. Dark Mode creates a darker screen that reduces eye strain (and saves battery). Here’s how to turn it on from a browser:
Open the Facebook website and log in.
Click the down arrow in the upper right corner.