The biggest feature in the iOS 13 update is a new system-wide Dark Mode. It’s a simple switch that you enable to turn system menus and Apple apps dark, but it can also darken third-party apps if their developers elect to support it. Trouble is, most haven’t yet, so half of your apps likely still have a blinding white background.

https://ios.gadgethacks.com/how-to/make-all-your-iphone-apps-work-with-ios-13s-new-dark-mode-0206479/