On the screenshot you can see nautilus with the eXperience theme. In the folder you can see the images that are used to draw the buttons and some other things in the theme. Notice that they most of the images are red. The color is changed by the engine to match the gtk colors.
The following shots show of some menu code:
No other engine does support this currently. Though I expect they will steal it at some point.
And if you really want to have a look at some boring and badly encoded video (it is scaled up ...): http://benjamin.sipsolutions.net/screenshots/side-detection.ogg
This shows off that the engine is able to detect, if there are two buttons next to each other in the toolbar. The idea is to make OSX like grouping of buttons with something like this. This needs more love before it can work correctly, and probably some support from the apps.