I've never seen code for CSS styles that takes the operating system into account, just never needed to. Can do it with PHP, maybe you could load a different stylesheet but unless the difference is very big, annoying and has negative effects. It might not be worth implementing.
Can you please confirm that you used the same browser on both computers?
Also if your aiming for your pages to always look the same to all visitors. Have you tried Explorer, Firefox, Opera and Chrome?
I'm just mentioning it because you could spend a lot of time getting it looking one way on MAC but it may still look different to 50% of your visitors using another browser.
Keep in mind that the issue may not be because it is a MAC but because of your MAC color profile for your display, different hardware, different software.
Good luck on this one.