function changeCSS(sURL){
var obj = document.getElementsByTagName("link");
for (var i=0, cnt=0; i<obj.length; i++)
{
if (obj[i].type == "text/css") obj[i].href = sURL[cnt++];
}
}

myW = window.screen.width;
if((myW) == 800){changeCSS(['http://www.kinfa.jp/jsimg/800_18.css','http://www.kinfa.jp/jsimg/base.css']);}
if((myW) > 1000){changeCSS(['http://www.kinfa.jp/jsimg/1100_18.css','http://www.kinfa.jp/jsimg/base.css']);}
if((myW) > 1100){changeCSS(['http://www.kinfa.jp/jsimg/1200_18.css','http://www.kinfa.jp/jsimg/base.css']);}