PlayStation.blog posted an official how-to, going meticulously step-by-step through the process of making original PS3 themes. It shouldn't be surprising, but there quite a number of icons used by the PS3, meaning a total XMB overhaul will take quite a bit of effort. If you're willing to take the jump, download the files you need here.
Unfortunately for Mac users, Sony's official theme program requires Windows. Considering how only 11% of you use Apple OS (according to our stats), it seems like Sony's neglect of the operating system will continue.