A more conventional way to switch to dark mode on iOS is to go to settings. Tap 'Display & Brightness' and tap either 'Light' or 'Dark' to choose which appearance you want. You can also tap the toggle underneath to set dark mode to come on automatically — simply go on 'Options' after turning it...

Link: How to turn on dark mode on all your apps and devices
via http://www.wired.co.uk