Art by Denys Tsiperko
Each pixel on an OLED screen is an “organic light emitting diode” that produces its own light. When the pixel is black, it isn’t producing any light. When the pixel is white, it’s producing light.
In other words, if you use a black background on an AMOLED display, your display will produce less light. This will help save battery power, squeezing more battery life out of your device.