var how_many_ads = 4;
var now = new Date()
var sec = now.getSeconds()
var ad = sec % how_many_ads;
ad +=1;
if (ad==1) {
url="css/bg01.css";}
if (ad==2) {
url="css/bg02.css";}
if (ad==3) {
url="css/bg03.css";}
if (ad==4) {
url="css/bg04.css";}

document.write('<link rel=\"stylesheet\" href=\"' + url + '\" type=\"text/css\">');