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	var $document = $(document),
	$element = $('#negro'),
	className = 'bg-black';
	
	$document.scroll(function() {
	  if ($document.scrollTop() >= 150) {
		// user scrolled 50 pixels or more;
		// do stuff
		$element.addClass(className);
	  } else {
		$element.removeClass(className);
	  }
	});