A modder developed a black-themed version of Google apps (he called this Black Google Apps) that is a much more battery friendly on OLED mobile phones. The application are not just "darker", they are inverted (see the image below).

This idea isn't new of course - when Microsoft developed the WP7 operation system they assumed most phones will sport OLED displays and this effected the user interface design.

Kyulux - Hyperfluoresence OLED emittersKyulux - Hyperfluoresence OLED emitters