Some AMOLED screen powered devices are able to “turn off” those pixels which are showing black colour. This app will let you to turn off some pixels in order to save your appreciate battery by showing very small black points.
As newer devices comes with very high density screens those points will be insignificant to the user´s sight and will be offset by lower spending battery

https://play.google.com/store/apps/details?id=com.anrapps.pixelbatterysaver