var ua = navigator.userAgent

var cssHTML = '<link rel="stylesheet" type="text/css" href="css/'
	if( ua.indexOf("Mac") != -1 ) 
		cssHTML += 'mac.css';
  else if( ua.indexOf("Win") != -1)
		cssHTML += 'win.css';
  else                               
		cssHTML += 'all.css';

  cssHTML += '">'

//alert(cssHTML)
document.write(cssHTML)