Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/contextual/contextual.api.php

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+<?php
+
+/**
+ * @file
+ * Hooks provided by Contextual module.
+ */
+
+/**
+ * @addtogroup hooks
+ * @{
+ */
+
+/**
+ * Alter a contextual links element before it is rendered.
+ *
+ * This hook is invoked by contextual_pre_render_links(). The renderable array
+ * of #type 'contextual_links', containing the entire contextual links data that
+ * is passed in by reference. Further links may be added or existing links can
+ * be altered.
+ *
+ * @param $element
+ *   A renderable array representing the contextual links.
+ * @param $items
+ *   An associative array containing the original contextual link items, as
+ *   generated by
+ *   \Drupal\Core\Menu\ContextualLinkManagerInterface::getContextualLinksArrayByGroup(),
+ *   which were used to build $element['#links'].
+ *
+ * @see hook_contextual_links_alter()
+ * @see hook_contextual_links_plugins_alter()
+ * @see contextual_pre_render_links()
+ * @see contextual_element_info()
+ */
+function hook_contextual_links_view_alter(&$element, $items) {
+  // Add another class to all contextual link lists to facilitate custom
+  // styling.
+  $element['#attributes']['class'][] = 'custom-class';
+}
+
+/**
+ * @} End of "addtogroup hooks".
+ */