This is a single-line Chrome extension:
document.querySelector('html').classList.add('__fb-dark-mode');
Messenger Web already has built in support for a dark mode, but no actual setting anywhere to turn it on. This extension runs the line above whenever a Messenger tab is loaded, enabling the "secret" dark mode.
- Download and unzip the repository
- Go to
chrome://extensions - Enable Developer Mode
- Hit
Load unpacked - Navigate inside the extension folder and click
Select Folder