Update to Drupal 8.0.0-beta15. For more information, see: https://www.drupal.org/node/2563023

core/themes/classy/templates/dataset/item-list.html.twig

@@ -12,10 +12,15 @@
  * - attributes: HTML attributes to be applied to the list.
  * - empty: A message to display when there are no items. Allowed value is a
  *   string or render array.
+ * - context: A list of contextual data associated with the list. May contain:
+ *   - list_style: The custom list style.
  *
  * @see template_preprocess_item_list()
  */
 #}
+{% if context.list_style %}
+  {% set attributes = attributes.addClass('item-list__' ~ context.list_style) %}
+{% endif %}
 {%- if items or empty -%}
   <div class="item-list">
     {%- if title is not empty -%}