The Sanisoft blog has a quick post covering the theming of a CakePHP application (via the functionality in the core of the framework).
Despite this there are continued request for information on how to use this functionality on the list and even googling for cakephp + theme does not really return anything definitive. Once you have figured out how to use theming, adding new themes is errr... ummm... a piece of cake (sheesh talk about cliches!).
They break down the use of themes into a few simple steps - adding the property to the controller, setting the application's theme and install the theme's files to the correct locations.