day=new Date()     //..get the date

x=day.getHours()    //..get the hour


if(x>=0 && x<7) {

   document.write('<style type="text/css">.container_12{background: white url(images/header_night.jpg) repeat-x}"></style>')

} else


if (x>=7 && x<17) {

   document.write('<style type="text/css">.container_12{background: white url(images/header_day.jpg) repeat-x}"></style>')
}
if(x>=17 && x<24) {

   document.write('<style type="text/css">.container_12{background: white url(images/header_night.jpg) repeat-x}"></style>')

}