After testing dark mode for a few months, Facebook Messenger rolled it out globally to all users in April 2019. Unfortunately, people had to manually turn it on or off in their app settings, meaning it couldn't follow your system preferences. This would have been particularly inconvenient for people who only turn on dark mode at night, for instance, as they would have to manually switch their Messenger settings as well. Thankfully, the app is starting to receive a new feature, allowing it to follow Android's global settings instead of manually having to fiddle with the app's color scheme.
A user on Reddit claims to have received an update to his Facebook Messenger's settings UI, adding an option to have the app automatically follow the user's Android system's settings when it comes to using dark mode or not. Interestingly, this option has appeared in app version 322.214.171.124.119 only for one of his accounts. I've tried enabling the feature on Beta version 3126.96.36.199.119, and it was nowhere to be found, hinting it could be activated through a server-side switch.
We hope it'll roll out globally soon, saving users the trouble of having to switch from one mode to another when using Messenger.