title: Change background color of selected items in "multiselect" dropdown box?
body: <p>I want to give the yellow color to the selected items in multiselect dropdown box. By default, it has gray background after selecting,
  How to do this in <code>HTML</code>, <code>CSS</code>.</p>
  <p>This question is about <strong>multiselect</strong> but for single select refer (Related but not duplicate of either): <a href="https://stackoverflow.com/questions/18091866">How to apply background-color to a selected option?</a></p>