Apple released iOS 13 for supported iPhones today, and one of the key new features is native dark mode. As usual, that means that a whole lot of apps are going to get updated on your iPhone today, and one of those apps is OneDrive.
And yes, the update is all about dark mode. The OneDrive app will automatically set itself to your system theme, so if your phone is set to dark mode, you'll get OneDrive in dark mode.
The feature has been in testing for a few days now, for those that are receiving TestFlight builds of the OneDrive app and are on the iOS 13/13.1 beta.
Microsoft actually announced the scope of its dark theme plans last month, and the plan was for it to start with Outlook, so expect to see that soon. The rest of the Office suite will be updated as well, although the one app that hasn't been mentioned is OneNote, which only has dark theme on Windows 10 right now.