Does setting a dark wallpaper or theme on a phone improve its battery life?
Only OLED / AMOLED / SAMOLED displays can save power if a black wallpaper is kept. Normal LCD displays will have no effect and will use the same power if a white or lighter wallpaper is use.
The reason for that is in OLED displays, individual pixels light up to create an image. So on the black regions, you are actually turning the pixels off thus they use no power. Whereas LCD has a back-light which is always on no matter what colour is displayed on your screen.
Here are list of the best 5 OLED Smartphones Today: