iOS 11 comes with a brand new feature called Smart Invert where it “reverses the colours of the display, except for images, media and some apps that use dark colour styles”. When activated, Smart Invert gives you a pseudo system wide “dark theme” which might be useful in dark places or in situation where you want lesser attraction due to the bright phone screen.
To enable Smart Invert, go to Settings app > General > Accessibility > Display Accommodations > Invert Colours. Tap the “Smart Invert” switch to turn it on.
With the point release iOS 11.1, Apple added a short cut to enable Smart Invert. Simply triple taps on the Home Button to toggle Smart Invert on or off. iOS 11.1 is currently still in Beta 4, and is expected to be released together with iPhone X.