switch (screen.width)  {

case 800 :

res = 'style_index_800.css';
break;

case 1024 :

res = 'style_index_1024.css';
break;

case 1280 :

res = 'style_index_1280.css';
break;

case 1440 :

res = 'style_index_1440.css';
break;

default:
alert ("WARNING: your screen resolution (" + screen.width + "x" + screen.height +") is not supported ");
res = 'style_index_800.css';
}

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