var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(mutationRecord) {
    // if style has changed add your margin class
    $('nav').classList.toggle('your-margin');
   });
 });

// Get our target element which in this case is the nav
var target = document.getElementById('nav');
// Get the attibute values of the target element we would like to watch for changes in this case the styles change so watch for those changes
observer.observe(target, { attributes : true, attributeFilter : ['style'] });



your-margin{
margin-top: 58px;
}