You mean like all the theme options we got in 1-6?
Steve would never have allowed it. /s
Steve would have no problem with two themes. It conforms nicely with Apple simplicity. No one is talking about multiple theme options, just two: dark/light. In fact if utilized within environmental context it is exactly the kind of "smart" feature Apple is known to adopt. Apple has a white iPhone and a black iPhone. Apple has single Home button press and double Home button press. Two themes would just be further refinement of Apple user-friendliness.
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I made these mockups weeks ago to see what a black theme would look like.
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A dark theme doesn't have to be all black. Use Garmin GPS devices as a reference (Garmin is what I'm most familiar with). When the evening arrives the devices change to a darker theme but it isn't "black." It uses darker color elements which actually aid in seeing the device better in the dark.
This is what Apple should do with its themes. My complaint with iOS 7 is the overuse of white. The overuse of black wouldn't be an improvment. My hope is that Apple will tone down the white but also add a dark theme as well.