userAgt = navigator.userAgent;

loadcss = " <link rel='stylesheet' type='text/css' href=' ";

if(userAgt.indexOf('Win') !=-1){
loadcss+=" ../win.css'> "; //Windowsに読ませるCSS
} else {
loadcss+=" ../mac.css'> "; //MACなどWindows以外に読ませるCSS
}

document.write(loadcss);