The thing is, Winterboard works pretty simply by replacing the .png files that have always made up the iOS UI. The issue is that much of the new UI does not consist of images at all, but vectors and dynamically generate content. The only things that really work the old way are app icons. For Winterboard to allow complete theming in the way it has in the past would/will require a rewrite from the ground up, with significantly more complex features. Even then, creating themes would become a significantly more complex process. Think about the complexity of the hidden SpringBoard settings...that's the type of thing we're looking at plus some.
I'm not saying it can't be done: I'm sure we'll eventually have theming capability of some sort for iOS7. But we should be prepared for it to look and work nothing like the Winterboard we've been using for years.
This post is horribly inaccurate
Firstly WB doesn't replace anything, your original files remain except WinterBoard hooks the runtime Bundles & your theme is used accordingly to show themed images/txt etc
iOS7 is full of pngs, so WB won't be just limited to AppIcons. I have faith Saurik can get it up close to where he had WB in iOS6. As long as he can hook these runtime Bundles & build support in it we should be fine.
Ill give u an example
Shared@2x.artwork in iOS6 contained 1178pngs & themed majority of the UI.
In iOS7 there are the equivalent two files
UIKit_OriginalArtwork.car - 912pngs
UIKit_NewArtwork.car - 167pngs
So 99 less from iOS6 to iOS7 responsible for the UI.
YES WB may require a rewrite but the method remains the same. "Hooking"
Shared@2x.artwork files were themed fine in iOS6 & below & if Saurik can make WB function the same with the .car files I don't see any reason why iOS7 can't be themed in similar fashion to iOS6.
With all the .car files these can be easily extracted via @0xced's iOS artwork extractor XCode project. Basically when you run his project it tears out all the pngs from the "core" UI files & makes a nice folder on your desktop with all the pngs in their respective folders. About 3900 pngs.
You can grab @0xceds extractor Xcode project here: https://github.com/0xced/iOS-Artwork-Extractor
Must be apple Dev registered & have Xcode & the iOS7SDK installed.
FYI WB is being worked on & tested for iOS7 with it being updated by Saurik.