Microsoft has been working on a new dark mode for Outlook.com for the past few months. The software maker is getting ready to roll it out to its web mail service users soon. Microsoft has already implemented a system-wide dark mode theme in Windows 10 and has been pushing forward with implementing it in other components such as File Explorer and their To-Do app. The beta version will receive dark theme update in the coming months and it should become available to users outside the beta program soon thereafter.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-s-dark-mode-for-outlook-on-the-web-will-launch-soon/