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Hey, if you have been using another device (phone, tablet or whatever) you might believe that the Nexus 7 uses AMOLED technology.
The Nexus 7 uses an LCD display which means using dark-themed apps will not save you any more power. This is also true of dark-themed wallpapers. Reason being, the back-light will always be turned on.
This means that if you are in a really tight pinch you can save additional power by disabling automatic brightness and then reducing it to the lowest level possible.
Is there a simple way to shut down optional processes (apps, whatever) that are not needed or core to operating the Nexus 7?
My battery life sucks...
If you want a better idea of what your Nexus is really doing to your battery I would suggest the GSam Battery Monitor App. It gives a nice snapshot of how much battery power is being used by each app on your phone.