What is a good design pattern for skinning an (iphone) app?

Question

I'm far far down the road of having built all my nice buttons and things in a XIB, and I had the sudden realization that people will pay $200 more for the lower quality laptop "Cause it's pink!" but I digress.

I need to somehow centrally control the colour scheme of my app, and change colours on the fly. I have a couple ideas, like maybe a singleton "Theme" object with a few kvo compliant properties holding theme colours and fonts, then have the view controller "listen" for changes, and re-paint everything when the theme changes.

Problems with this so far: I'd need to have a pointer to every single UI object, including things like table view cells - which seems like a pain. Another possibility would be to subclass all my themed UI objects, maybe get them to register as observers on their own, but that makes me wonder about the overhead needed for this, maybe KVO isn't even the way to go here, I don't know.

So I'm wondering if anyone could share what they've done in the past, what works and what leads to big problems?

Thanks!

Update

I ended up going with a singleton object called SkinDispatcher, which only contains a lot of properties that are meant to be observed by UIView subclasses. Then I made quick and dirty subclasses of UILabel, UIButton, UITextField, anything else I happened to use.

These subclasses each looked up their tag number and used it to register for changes to the applicable fonts and colours, in awakeFromNib.

Next I made a class specifically for loading a style, which essentially opens up a plist file full of keys holding font names and R|G|B|A colours, reads them in and sets them to the applicable property in SkinDispatcher, as determined by key name. The last step (yet to be tried) is to set an observer on the StandardUserDefaults key for skins, props to the answerer for that idea.

Answer 1

You already mentioned a possible approach, namely using KVO. ANother possibility is to register for a specific notification, and react accordingly when the notification arrives in your view controller.