Microsoft has been working on a dark mode for Outlook on mobile devices for a year now and it looks like it is just about here. Both Outlook apps for Android and iOS will be getting a new switch that will allow you to enable dark mode.
Microsoft will also be adding this mode to the other mobile apps in its suite (Word, Excel, PowerPoint, SharePoint, OneDrive, Planner, and To-Do) for the launch of iOS 13.