Front runners for now... BLACKBERRY ring a bell? Technology is all about competition and innovation (which will burst at some point). Customization is going to be the king, and this is where Apple falls short. You can at minimum change the color scheme in WP.
I'm not getting dragged into this argument outside of this...people have said for years that Apple was "falling off" and falling "behind", they said that people are losing interest in Apple products because they aren't more "customizable" (with things like themes, as you mentioned)...people say all these things, but year after year after year, the iPhone is the best selling smart phone pound for pound compared to any individual smart phone on the market.
Blackberry's downfall wasn't lack of customization or feature sets that were too narrow compared to the rest of the market. Blackberry failed because they made piece of junk phones on a decade old, stagnant OS and when they finally saw the light, they brought a whole new experience to the table with modern devices and a new OS, but it was too little too late. Blackberry never enjoyed the kind of consumer device success that Apple did, even in their golden days...they enjoyed success and being a front runner for quite a long time, but nothing like what Apple has across the spectrum, not just with the iPhone and iOS.
There's nothing "innovative" about customizable themes, I mean you have to face it, Android phones have offered near limitless native customization options on their devices for the better part of 5 years now, but they still can't compete with an iPhone launch.
Again, you can disagree all you want to but the proof is in the pudding...Apple doesn't need to implement things like customizable themes to be "competitive", you just feel they lack competitiveness because YOU want theme options. It's not as big a demand as you think my friend...and even saying that, the iPhone isn't locked out of theme options in the first place, you just have to jailbreak the device in order to open up those options.
So not only are the options unnecessary for competitiveness, but in the end, they're still available to iPhone users if they choose to do so. If you go and read polls about jailbreaking though, I think you'll see that a pretty staggering majority of iPhone users don't care about this customization demand, and don't jailbreak their devices because of such.