https://techcrunch.com/2019/05/07/android-q-scores-a-system-wide-dark-theme/

Google is bringing a system-wide dark mode to Android Q. It’s called Dark Theme and it’s exactly what you would expect, changing white page elements to solid black across the OS for friendlier night-time viewing.

You’ll be able to activate dark mode by tapping a dedicate tile in Quick Settings or it can be auto-triggered when you turn on battery-saver mode. The company says the mode “it will help you save battery,” highlighting how “Dark Theme” will fire up less pixels on your OLED device.