iOS 7 was more than just a redesign, it was a re-imaging of what mobile interfaces should look and act like. Part of that involve 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. 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.
A dark mode certainly isn't as easy as flipping a bit. Right now, in iOS, you can go to Settings > General > Accessibility > and toggle Invert Colors to on. That'll make the white turn to black but it'll also make the blue turn to orange, the green turn to purple, and every color switch to its opposite. Including your wallpaper, your photographs, your... everything.
Because there's no proper dark theme, people use Invert Color as a work around for everything from surfing in a darkened living room to texting in bed.
For the Apple Watch, by contrast, Apple recommends 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, that's still not a theming function.
Apple's iBooks app for iOS does have a proper dark theme, though. 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 be, so they remain the proper colors.
If the same logic could be applied to all of Apple's apps, and if a night theme could be set to auto system-wide, it would go a long towards making the iPhone more usable in the dark.
If a UIKit framework could also be created for developers, they could tie into the same system, and set their own alternate interface colors for when the night theme kicks in, it 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.