CGP Gray posits Apple should add a "dark mode" to iOS, a la OS X Yosemite:
I'm still holding out hope for Apple to, one day, introduce a true system-wide dark mode for iOS that apps can opt into participating with their defaults. (Apple already has done this with OS X, sort of.) But until that day comes, I ask developers of text-heavy apps: please consider including a dark mode for your app not just because it's a night-use feature but also because, for some of us, it's an accessibility feature.
As Gray says, having a dark mode is a matter of contrast and legibility. (via Marco Arment)