The new theme, called Pastel Blue for OS X, was created by Michael Rabe and serves, if nothing else, as an example of what can be done to the OS X interface. According to ResExcellence:
ResExcellence has posted the first entirely new theme for MacOS X. This is the first theme that will entirely replace all the Aqua elements in MacOS X. Theme artist Michael Rabe worked for months decoding the format of the Extras.rsrc file, and readers can now download the results.
You can find the new theme at the ResExcellence Web site.