Question

How can I highlight the div if it is visibile?

$('#add_dom').click(function(){
  $('#new_dom').slideToggle("slow");      
  return false;
});

Answer 1

After your slide ends, this function will check if it is visible and add or remove a class of highlight.

$('#add_dom').click(function(){
  $('#new_dom').slideToggle("slow",function(){
    $(this).is(':visible') ? $(this).addClass('highlight') : $(this).removeClass('highlight');
  });      
  return false;
});

Hope that helps 🙂