Bloard is very simple and features an on/off switch in the Settings app, letting you choose to have the dark keyboard, or let the OS use the standard white one.
Tired of white keyboards in iOS 7? Install Bloard! Bloard while turn your white keyboard into a nice light black keyboard easier on the eyes. You can turn it off anytime from the stock settings app. No settings to configure. Works on every app with a keyboard.
The tweak can be downloaded for free from the BigBoss Repo on iOS 7 devices.