Question

I want to specify two slightly different background colors, one for Mac OS, one for Windows.

Answer 1

there is no property to specify OS used to view webpage, but you can detect it with javascript, here is some example for detecting Operating system :

var OSName="Unknown OS";
if (navigator.appVersion.indexOf("Win")!=-1) OSName="Windows";
if (navigator.appVersion.indexOf("Mac")!=-1) OSName="MacOS";
if (navigator.appVersion.indexOf("X11")!=-1) OSName="UNIX";
if (navigator.appVersion.indexOf("Linux")!=-1) OSName="Linux";

console.log('Your OS: '+OSName);

got it?, now you can play with document.write to write download stylesheet for specific Operating system. 🙂

another example, i assumed that you are using jquery.

if (navigator.appVersion.indexOf("Win")!=-1) 
{
  $(body).css('background','#333');
} else {
  $(body).css('background','#000'); // this will style body for other OS (Linux/Mac)
}