Outlook.com is a web-based suite of webmail, contacts, tasks, and calendaring services from Microsoft.
Dark mode changes the default bright background color in Outlook.com and Outlook on the web to a darker color, which is easier on the eyes in low-light environments or if you prefer less bright interfaces.
See also: Dark mode in Outlook.com and Outlook on the web | Microsoft Office
This tutorial will show you how to use a light or dark mode theme in Outlook.com and Outlook on the web.