https://techcrunch.com/2019/04/15/ios-13-could-feature-dark-mode-and-interface-updates/

According to a report from 9to5mac’s Guilherme Rambo, the next major version of iOS for the iPhone and iPad will feature many new features, such as universal dark mode, new gestures, visual changes for the volume popup and more.

Dark mode should work more or less like dark mode on macOS Mojave. You’ll be able to turn on a system-wide option in Settings. Apps that support it will automatically switch to dark mode the next time you launch them. Let’s hope that third-party developers will support that feature. Otherwise, it would be a bit useless if Facebook, Instagram, Gmail or Amazon still feature blindingly white backgrounds.