var isIE  = (navigator.appVersion.indexOf("MSIE") != -1) ? true : false;
var isWin = (navigator.appVersion.toLowerCase().indexOf("win") != -1) ? true : false;

if(isWin && isIE){
//windowsかつIEの場合・
	fName = "ie_style.css";
}else if(!isWin){
//Macの場合
	fName = "mac_style.css";
}else{
//上記以外(Fire Fox)・
	fName = "style.css";
} 
document.write("<link rel='stylesheet' href='"+fName+"' type='text/css'>");