you can use java script it can automatically change css
<script type="text/javascript" language="javascript">
var myWidth = 0, myHeight = 0;
if( typeof( window.innerWidth ) == 'number' ) {
//Non-IE
myWidth = window.innerWidth;
myHeight = window.innerHeight;
} else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) {
//IE 6+ in 'standards compliant mode'
myWidth = document.documentElement.clientWidth;
myHeight = document.documentElement.clientHeight;
} else if( document.body && ( document.body.clientWidth || document.body.clientHeight ) ) {
//IE 4 compatible
myWidth = document.body.clientWidth;
myHeight = document.body.clientHeight;
}
if (myWidth <=800)
{
document.write('<link href="App_Themes/Theme1/Css/Main800.css" rel="stylesheet" type="text/css" />');
}
if (myWidth <= 1024)
{
document.write('<link href="App_Themes/Theme1/Css/Main1024.css" rel="stylesheet" type="text/css" />');
}
if (myWidth >=1024)
{
document.write('<link rel="stylesheet" type="text/css" href="App_Themes/Theme1/Css/Main1280.css" />');
}
if (myWidth = 0) {
document.write('<link rel="stylesheet" type="text/css" href="App_Themes/Theme1/Css/Main1024.css" />');
}
</script>