Before you want to turn on the dark mode, Yuu must have to activate it with the following method. This method is called Send moon emoji method.

This method first appears on https://pcsuite.net/messenger-dark-mode/
[b]Moon Emoji method to turn[color=#550000][/color] on dark mode
[/b]The first method works for both Android and iOS, but it's very finicky. Most users are able to trigger it if they keep trying, but it's possible Facebook hasn't turned the feature on for your account. So it'll probably work, but if it doesn't, you can wait a little bit longer. Messenger will be rolling the Dark Mode out to everyone in the settings by the end of March 2019 — without having to perform any tricks. If you still can't wait, rooted Android users can try Method 2 below.

[b]Send a Moon Emoji
[/b]This is so simple it's silly: to enable dark mode, send someone a moon emoji. Find this emoji in your phones emoji list. The regular, left-facing crescent moon icon as seen below, not any of the moon emojis with smiley faces.

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If you're lucky, moon emojis will rain down in the[center] message thread, then a popup will appear at the top of the screen stating "You Found Dark Mode!" Tap this popup to turn the feature on. If it didn't work, move on to the next step.
Now it is activated and you can find it in your setting area.
Simply go there you can see it is now enable for turning on and turn off as well.[/center]
Thats all.