https://www.bleepingcomputer.com/news/security/dark-mode-rolling-out-now-to-outlookcom-users/

Earlier this month, we reported that a Dark Mode was coming to Outlook.com. Well, it is finally here and currently being slowly rolled out to Outlook.com users right now.

Dark Mode is currently only available in the beta version of Outlook.com, so be sure to be using the beta before checking to see if Dark Mode is available for your account. To check if Dark Mode is enabled in Outlook.com, go into Settings > Quick Settings > Dark Mode.