First make sure your phone has an AMOLED screen. This app will only save battery life if you have an AMOLED display. See link below for checking.
If you have one then you can actually save your phoneâ€™s battery life simply by applying our black wallpapers.
On an Amoled screen black colors are generated by turning off the backlight/pixels, creating a dark pixel. A pixel that is turned off does not use any power. Therefor the more black, the more battery power you safe.
Use our black wallpapers and turn your homescreen dark. We have 6 dark tone wallpapers. If full black is too dark you can choose a lighter wallpaper which will let in a little backlight.
"Black wallpaper is the best battery-saving trick you'll ever find" - AndroidPit
* XDA Developers did an experiment and figured out that you can save about 6% battery life/hour at 20% brightness by using a black wallpaper on an AMOLED display, and about 8%/hour at full brightness: http://forum.xda-developers.com/showthread.php?t=660853 . These results are based on an always-on screen, but the basic theory is sound.
Use the list on this website to check if your phone has an AMOLED display http://en.wikipedia.org/wiki/AMOLED