Many apps and popular websites now count a dark mode feature among its many platform tools. The optional view setting changes the background from white to black, and text from black to white.
Dark mode is popular among many people for several reasons. For starters, it consumes less battery because displaying darker pixels on an LED screen takes far less power than displaying white ones.
There are also those who find that it is less straining on the eyes, and many believe that the reduced blue light helps them sleep better - although the jury is still out on the science behind that.
Step 1 :- First of all, login with your Facebook account on the desktop.
Step 2 :- On the top-right corner, click on the down arrow button, as shown in the screenshot below.
Step 3 :- From the list of options, select "Display Preferences".
Step 4 :- Under the Display Preferences, you will find the Dark Mode option. Simply, select the "On" to enable the dark mode.