Hey Christian, try adding this code to your
Custom Code -> Body Custom Code area:
<script type="text/javascript">
jQuery(document).load(jQuery(window).bind("resize", listenWidth));
function listenWidth( e ) {
if(jQuery(window).width()<767)
{
jQuery("#sidecol_b").remove().insertAfter(jQuery("#content_remainder"));
} else {
jQuery("#sidecol_b").remove().insertBefore(jQuery("#content_remainder"));
}
if(jQuery(window).width()<767)
{
jQuery("#sidecol_a").remove().insertAfter(jQuery("#content_remainder"));
} else {
jQuery("#sidecol_a").remove().insertBefore(jQuery("#content_remainder"));
}
}
</script>