One of the methods he uses to create the background is
stretchableImageWithLeftCapWidth:topCapHeight:, which splits an image into several pieces and repeats others to create a nice, repeating background with special treatment on the edges.
With the work I did recently playing around with webkit scrollbars, I knew that -webkit-border-image worked very similarly and threw together a quick sample of how it could work:
This sample sets the “cap” width on both the right and left to 15px, but you could easily change that to whatever your image required. Here’s a working demonstration. I’ve only tested the WebKit implementation of border-image, so your milage may vary with Firefox’s implementation.