select2.js multiple

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/select2/3.2/select2.min.js"></script>
<link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/select2/3.2/select2.css">
<select id="dropdown">
    <option>Item 1</option>
    <option>Item 2</option>
    <option>Item 3</option>
    <option>Item 4</option>
  </select>

CSS

.select2-results .select2-highlighted {
  background: gold;
  color: purple;
}

JavaScript

$(document).ready(function() {
  $("#dropdown").select2();
});