iOS 7 was more than just a redesign: It was a reimagining of what mobile interfaces should look and act like. Part of that involved a lot of flat, white space. A lot, a lot. It made the iPhone and iPad look bold and bight, which is fine for everyday use, but when it comes to low-light situations — like the home theater or the bedroom — it can be a little too bold and bright. If you're not careful, it can get you yelled at and hit with a pillow. It's enough to make me wish for a dark theme in iOS 9.
Right now, because there's no proper dark theme, some people use the "Invert Colors" accessibility option as a workaround for everything from surfing in a darkened living room to texting in bed.
That makes white turn to black... but it also makes blue turn to orange, green turn to purple, and every color switch to its opposite. Including your wallpaper, your photographs, your everything.
For the Apple Watch, in contrast, Apple recommends a black background for the interface. That could be because it's widely rumored the company is using OLED displays and black performs better on them, or it could be because the bezels are black and it will make apps look bigger. Either way, though appreciated, it's still not a theming function.
Apple's iBooks, however, sports a proper dark theme: You can manually switch it on, or set it to turn on automatically at night. Rather than invert colors, it sets them appropriately for low-light conditions. It makes the white background black, the black text light gray, and blue and green tint colors the right shades of gray. Importantly, it leaves pictures, photos, and other graphics as they were, so they remain the proper colors.
Some App Store apps currently include their own theming options. In Fantastical you can toggle a light mode on in the app's settings. In Tweetbot you can swipe down with two fingers to switch between light and dark modes. Many apps don't provide the option, however, and even those that do vary in how it's implemented. That leads to an inconsistent experience.
If Apple could provide a night theme for all of its apps, it would go a long way toward making the iPhone more usable in the dark. To take it a step further, if Apple could provide a night theme framework in UIKit for developers, then App Store apps could tie into the same system and set their own alternate interface colors for when it kicks in.
We could put it on automatic in Settings > Display and Brightness, or toggle it on or off from Command Center at any time. Either way, a systemwide night theme would make the iPhone damn near perfect in the dark.
Note: I've filed this feature request with Apple. It can be found at rdar://19760524.