Starting with Egg Mode means that only a small percentage of people know this way, and it also shows that Messenger's dark mode is not fully prepared.
To enable Dark Mode, all you have to do is send a "New Moon" image emoji to any conversation in the chat list. Doing so will trigger a smiling new moon emoji animation, for some reason, then you will need to manually close and reopen the app. After restarting the app, go to the settings page and see the mode switch at the top of the settings list.
For those who prefer to use the app at night or prefer dark mode, Dark Mode is a popular addition to Messenger. The black theme is also clearly beneficial on OLED/AMOLED displays because it extends battery life. However, it seems that the completion of the dark mode is already very high, I believe it will be officially released soon.
We can confirm that this method works for Android apps, but it is reported to work for iOS.