styler.html

<script>
  init({
    title: 'the styler',
    desc: 'Use <code>styler</code> to custom the item style',
    links: ['multiple-select.css'],
    scripts: ['multiple-select.js']
  })
</script>

<style>
select {
  width: 500px;
}
</style>

<select multiple="multiple">
  <option value="1">January</option>
  <option value="2">February</option>
  <option value="3">March</option>
  <option value="4">April</option>
  <option value="5">May</option>
  <option value="6">June</option>
  <option value="7">July</option>
  <option value="8">August</option>
  <option value="9">September</option>
  <option value="10">October</option>
  <option value="11">November</option>
  <option value="12">December</option>
</select>

<script>
  function mounted() {
    $('select').multipleSelect({
      styler: function(value) {
        if (value == '1') {
          return 'background-color: #ffee00; color: #ff0000;';
        }
        if (value == '6') {
          return 'background-color: #000; color: #fff;';
        }
      }
    })
  }
</script>