You have to change the target on something different than #openModal. You can import jQuery, put this in <head>
section:
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
and add this code right before </body>
tag:
<script type="text/javascript">
$("#openModal").click( function() {
if (!$("#openModal>div").is(":hover")) document.location.href='#';
});
</script>
So when user clicks on the overlay the target will change on '#' and the modal will be closed.