Add Class Based On Z-index

$('.main-stack').each(function () {
    if ($(this).css('z-index') == '5') { // Change here.
        $('.main-stack .card')
            .addClass('ui-front top-card')
            .removeClass('flipped ui-draggable-disabled ui-droppable-disabled')
            .draggable({
                disabled: false
            });
    }
});