Item

<fine-item id='myItem1'
  config='{
    "data": "Vietnam"
  }'>
</fine-item>
  
<select class="form-control" onchange="changeTheme(this);">
  <option>default</option>
  <option>flat</option>
  <option>dark</option>
</select>

<script type="text/javascript">
  function changeTheme(select) {
    var el = document.getElementById('myItem1');
    var theme = select.selectedOptions[0].textContent;
    el.set('theme', theme);
  }    
</script>