According to some users’ comments, after the update, some accessibility options seemed to be missing, including Dark Mode. We were running version 319.0.0.38.119 (292363515) on iOS, which according to the Apple App store, was last updated six days ago, and we had no issue with our Dark Mode.
We can confirm though that the problem seems to be plaguing version 320.0.0.36.122 (293873823). We downloaded the new iOS version on several iPhones and were able to replicate the issue.
Here is the way to restore dark mode
First, uninstall Facebook, then reinstall.
If Dark Mode isn’t accessible, force quit the app by sliding you finger up slightly from the bottom of the home screen, then swipe up on the Facebook app.
Next, go into your iPhone’s settings and scroll down to the app section. There, select Facebook. In the Facebook app setting, turn on “Upload HD video” and “Upload HD photos.” Weird as it is, that has gotten us Facebook Dark Mode back on at least three different iPhones.
What happened to Facebook Dark Mode setting?
Facebook company spokesperson acknowledged the issue in a statement to TechRadar: “We’re aware that some people are having trouble accessing certain features on Facebook’s Android app. We’re working to get things back to normal as quickly as possible.” That doesn’t say anything about the iOS app, but fortunately there at least appears to be a way to get it back on iPhones and iPads.
Jason 
Sean Lee 
T4TeCH