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How to get dark mode on desktop
Facebook is now allowing people to try The New Facebook and its dark mode from a desktop. Just login to Facebook on your computer, go to your Settings drop-down menu, and select the option toward the bottom that says "Switch to New Facebook." You will be brought to the all-new look and shown a card detailing the key changes. Then, you will be allowed to choose the dark mode.
https://www.pocket-lint.com/apps/news/facebook/150761-the-new-facebook-update-how-the-all-new-design-looks-and-works