4
The three CSS in your each theme folder are:
style.css
styleMAC.css (No need)
styleNN.css (No need)
All you do it's to use the style.css and the site will be compatible with every browsers on the any operation system.
Check out my site, I made myself a complex theme, it's hard to do it but it work on every browser I have tested. So general and basic theme will be working propertly on any browser unless you have some special css code which only compatible to a specific browser.