https://arstechnica.com/gadgets/2018/07/outlook-com-the-latest-site-to-copy-ars-offer-dark-mode/

The most popular feature request for Microsoft's Outlook.com has been to add a dark/night theme that replaces the eye-searing white with a calming dark grey, making it less visually traumatic to check your e-mail after dark. Microsoft has now announced that this is coming soon, as spotted by The Verge.

The new theme will be based on a special dark theme that was offered last Halloween. Since then, it's apparently been "redesigned multiple times" and the company claims that it will be "the best Dark Mode of any leading e-mail client."