The set of easy steps you'll find below is a trick that will allow you to try the night mode in Messenger Facebook, which can completely change your experience.
Step 1: First, run Facebook Messenger on your iOS or Android device, allow it to download all your conversations and other details, then choose a conversation to work with.
Step 2: After entering that conversation, go to the Send a message window and click the icon that lets you choose between GIFs, emoji, or posters. This should appear as a smiley face icon on the right side of the text input field.
Step 3: Select the EMOJI option and scroll through the list of emoticons until you reach the Animals and Nature section, then select and send the Moon icon.
Step 4: Once you have sent this emoticon, you should see an animated image of the moon falling like rain from the top of the screen, followed by a notice as shown below. "I found the night mode!" Then press the Turn On Settings button and you will be taken to a page Settings. Here, just switch the night mode switch in Messenger to Facebook to run, and everything will be ready!