By default Umbraco comes with a collection of prevalue editors, for settings in a Grid layout. One of them is multivalues. I tried to use it to add more then one class to a class attribute, but it renders the json string and not the added values in the multivalue editor. I guess there is some modification to do in a render file, but I can't figure out where this render-files are for settings. I can only find the render files for grid editors.

It renders this:

<div class="[
  {
    " value":="" "white-bg"="" },="" {="" "value":="" "about-us-section"="" }="" ]"="">
        <div class="container">
            <div class="row clearfix">
                    <div class="col-md-4 column">
                        <div>
                            <p>asdfasdf</p>
                        </div>
                    </div>            
                    <div class="col-md-8 column">
                        <div>
                        <p>adsf</p>
                        </div>
                    </div>        
        </div>
        </div>
</div>