I plan to gradually post some examples of customizing the appearance of Cocoa controls, sometimes called skinning the user interface. You'll be able to change the look by supplying a set of image files, representing the parts or states of the controls. The images supplied with the sample code are ugly placeholders. The source code is released under the zlib/libpng license, and hence can be used for free even in commercial code.
In most cases, we alter the appearance of a control by subclassing the cell class, not the control class.