https://thenextweb.com/facebook/2019/03/04/enable-dark-mode-in-messenger-by-sending-an-emoji/

At its developer conference last year, Facebook promised that it would bring dark mode to Messenger; it looks like the company’s rolling out the feature at last. However, As Android Police first noted, instead of enabling it through the settings menu, you’ll have to rely on an easter egg to switch on dark mode – both on Android and iOS – and it involves sending an emoji.

Here’s what you’ll need to do.