Once you've gone inverted, it's hard to go back to black text on a white background. Of course, applying this theme of altered colours is easier in some places than others, with Apple devices being one of the more trickier propositions. Fortunately for those with jailbroken gadgets running iOS 7, there's a pay-for option that will do a decent job of darkening your mobile experience.
The tweak you're after is called Eclipse and it's available from the BigBoss repository on Cydia for $US0.99. Eclipse is configurable via the Preferences menu, but the core functionality is designed to apply a dark theme across the system -- if possible.
It doesn't work in some places, but the tweak is being fine-tuned and should hopefully become more compatible as updates are released. For now, it covers all the important areas, with the only downside being that you need a jailbroken device to take advantage of it.