I develop all my sites -- Zen and others -- here on my Mac. I can't think of anything that would cause your store to look very different on a Mac. If you've specified fonts available only on your system, then that will cause problems, for Mac or Win users without those fonts.

But I can't help you track down the problem, since when I went to your store, your log-in code trapped Firefox in an infinite loop and I had to force quit the process.

Rob