<script type="text/javascript">
d=document; today=new Date(); hour=today.getHours();

if(hour>=0 && hour<6) d.write("<style>body{background-image:url('http://indirizzo_sfondo_1')}</style>");

else if(hour>=6 && hour<12) d.write("<style>body{background-image:url('http://indirizzo_sfondo_2')}</style>");

else if(hour>=12 && hour<18) d.write("<style>body{background-image:url('http://indirizzo_sfondo_3')}</style>");

else if(hour>=18 && hour<21) d.write("<style>body{background-image:url('http://indirizzo_sfondo_4')}</style>");

else d.write("<style>body{background-image:url('http://indirizzo_sfondo_5')}</style>")
</script>