Add"Theming" Ability in App

Don't. How much effort is it? How many more buyers will you get? How many bugs will you introduce? If someone watches a friend using your app, will they recognise it on the App Store? Do _one_ nice design.

 

Gnasher is probably right, it adds a lot of work for you and most users might think it's cool - but wouldn't miss it if it never was there.

I did spend some time doing a skinned app that used swfs for the UI. Probably a performance hog, but was cool to play with. I never tried with a very animated swf, that could have been fun.

 

I haven't really played with Interface Builder or Cocoa in a while (I'm learning Qt and usually work in Win Forms/C# & VB.net) but on the dev platforms I'm familiar with its rather trivial to set colors or overlay bmps and other images over your window object. I think that stuff is cool for music players web browsers and similar apps but skinning something like Photoshop or Final Cut Pro wouldn't make much sense .