Facebook is now introducing a dark mode on Messenger. The Facebook Messenger app seems to have slimmed down with more focus on the chat function. The dark mode is basically a display mode wherein the screen background is dark and the text and other features seem to be light.
One of the ways to test this mode is to start the chat thread and send the person moon emoji. After sending the chat tap on the sent emoji icon and you shall get a message saying that you have found the Dark Mode.
You can then go to the messenger settings and then toggle the Dark Mode switch to on.