How to activate Smart Invert in iOS 11

The new "Smart Invert" feature in iOS 11 is a dark mode, but smarter. 

For a while, Apple has been trying features to ease the nighttime burden on our eyes when they're staring at screens, with greyscale mode in iOS 8 and Night Shift in iOS 9. Smart Invert, iOS 11's go at eye relief, inverts some colors, but leaves images, videos, and some apps untouched, aiming to dampen your screen's glow without leading to the really weird image contortions that we saw with previous iterations of the feature.