It is a known fact that AMOLED displays can display a truer darker black. The reason for this is that to represent black they simply turn off pixels. The screens have no back lighting since light is produced when current passes through the organic material the display is made of. This means you can save some battery life by doing things like using a black wallpaper.
You could potentially save even more battery by using inverted apps. An inverted app is a modified (themed) version of an app which replaces the white or lighter elements with black or darker elements. Not only will it probably save some portion of battery life. It also just looks really cool!
Team Black Out is a team of themers and developers who specifically maintain a list of inverted apps. They do a pretty good job of keeping their list of apps updated as well. You can actually download and install their versions of some of your most used apps from within the TeamBlackOut app. Once you have downloaded the app you want just flash it in TWRP. Yes you will need to have a custom recovery installed. Head to the link below to grab this app.
TeamBlackOut app Download