Microsoft has updated the Outlook Mail and Calendar apps with support for the Dark theme on Windows 10 Mobile.
The changes are included in the Personalization Settings of the apps, and now lets you choose between Light theme, Dark theme and following the accent colour. The actual Settings button has now also moved from the hamburger menu to the ellipses menu.
The update takes the app to 17.6216.42272, and also appears to have removed the button in Outlook Mail to open Outlook Calendar.
Find the updated app in the Windows Store here.