OP obviously means the ugly gray color of the dock has been removed. But how does this work? Is it the specific size of the image file used?
Edit: no, it claims to be caused by a 1px wide background image. It does work, whereas creating a screenshot of the dock color, then using that as the background doesn't work, the dock just gets lighter to compensate.
Too bad it messes with the Today view colors when you slide left from the Home screen. Oddly, if you pull down Notifications, and then slide over to Today, it looks normal.