Is it possible to create a Mac OS specific CSS to fix font difference?

I'm afraid that browser/os sniffing is your only option. CSS itself has no knowledge of OS nor do i have ever heard of a css hack that targets osx specifically.

This is the easiest way for me to detect OSX and add the OSX class to the document body so you can override css styles for OSX specifically.

if(navigator.platform.match('Mac') !== null) {
    document.body.setAttribute('class', 'OSX');
}