A CodePen by Jacek Dominiak.

index.html

<select multiple='multiple' id="select">
  <option>one</option>
  <option>two</option>
  <option>three</option>
  <option>four</option>
</select>
<div id="up"></div>
<div id="down"></div>

style.css

#up, #down {
  width: 0;
  hegith: 0;
  border-left: 5px solid transparent;
  border-right: 5px solid transparent;
  margin: 10px;
}

#up {
  border-bottom: 5px solid #111;
}

#down {
  border-top: 5px solid #111;
}