It’s kind of funny/sad to see how Apple operates with APIs. None of these concepts are really “new”: they were there, in Carbon, prior to Mac OS 8, in the days when that OS was meant to support really elaborate themes.
The original theme APIs were huge, and 3rd parties were expected to adopt them as needed (e.g. using appropriate theme selectors for fonts and background patterns). For about a year, things looked really cool: you could create a great Platinum-esque UI for any custom control, and it would fit right in.
Then, they threw it ALL away for Mac OS X, ironically right about the time when an appearance-agnostic API would make a lot of sense. They didn’t even bother to update their API implementation to ease migration: really trivial changes that would have been helpful, like “update the damned font/color/pattern value returned for X to resemble Aqua”, were not done. This meant that an app full of seemingly-appearance-agnostic code would look like crap under Aqua, and you had to rip it all out.
And it’s not like Mac OS X didn’t change its look. Heck, for a long time you could find 3rd party app bundles chock full of images that were there just to implement the Apple-Look-and-Feel-of-the-Week, because the SDK didn’t offer any reasonable way to do half of what they came up with.
So yeah, 14 versions later, the Mac wants to be appearance-agnostic again. Sounds great. My advice: always have the option of doing your own UI as a backup.