if(navigator.userAgent.indexOf("Mac")!=-1){
    document.write("<LINK REL=STYLESHEET HREF=http://www.sakaeya-yuwaku.com/test_img/css/style_mac.css TYPE=text/css>"); 
} else {
    document.write("<LINK REL=STYLESHEET HREF=http://www.sakaeya-yuwaku.com/test_img/css/style_win.css TYPE=text/css>"); 
}