Feeling complex at starting after knowing and doing mistakes i felt it was very simple: i followed the below steps to achieve the above jquery rounded functionality in my theme:
1.added the below code in portal_normal.vm file in section
<script type="text/javascript" src="/html/js/jquery/jquery.js"></script>
<script src="$javascript_folder/jquery_roundcorner.js"></script>
2.created the jquery_roundcorner.js file under _diffs/js folder
added the below code: for adding jquery round corner functionality in banner
$(document).ready(function() {
$('#banner').corner();
});
3.removed the border-radius property in banner of custom.css under _diffs.