If you're willing to go as far as leaving the walled garden, you can get e.g. Twilight on Android. Not what you asked for of course but this is actually a good example of where the closed nature of iOS hurts users.
Such a shame, but I don't think APIs are available to developers for this purpose
Evasi0n7[1] runs as an application on your PC or Mac with your device connected, jailbreaks the device and installs Cydia. It does not affect anything in the regular Apple App Store, it merely lets you install additional programs including tweaks that customize the interface. Perhaps interesting, f.lux is featured on the front page of Cydia, so you don't even have to search for it.