if(navigator.userAgent.indexOf("Mac") == -1)
{
if(navigator.userAgent.indexOf("Win") != -1)
  {
  document.write('<LINK HREF="pc.css" TYPE="text/css" REL="stylesheet">');
  }
else if(navigator.userAgent.indexOf("X11") != -1)
  {
  document.write('<LINK HREF="unix.css" TYPE="text/css" REL="stylesheet">');
  }
}