I see why Apple decided not to go with themes in iOS. They want to build the best experience they can for everyone, and it's crucial to represent their devices with their signature look and feel. They want it to be instantly recognisable around people. Based on some themes on Android I've seen, I doubt that some people are smart enough to use a decent theme.
It wasn't a problem when Steve Jobs was around, but I don't want to brag about it. What they should do now is to listen to their community, if they aren't able to build a superior look and feel.