Update to drupal-org-drupal 8.0.0-rc2. For more information, see https://www.drupal.org/node/2598668

core/themes/seven/css/components/buttons.css

@@ -127,7 +127,7 @@
 /**
  * 1. Use px units to ensure button text is centered vertically.
  */
-.no-touch .button--small {
+.no-touchevents .button--small {
   font-size: 13px;
   font-size: 0.813rem;
   padding: 2px 1em;  /* 1 */

core/themes/seven/js/mobile.install.js

@@ -1,6 +1,6 @@
 (function () {
 
-  "use strict";
+  'use strict';
 
   function findActiveStep(steps) {
     for (var i = 0; i < steps.length; i++) {

core/themes/seven/js/nav-tabs.js

@@ -7,7 +7,7 @@
  */
 (function ($, Drupal) {
 
-  "use strict";
+  'use strict';
 
   function init(i, tab) {
     var $tab = $(tab);

core/themes/seven/seven.theme

@@ -5,7 +5,6 @@
  * Functions to support theming in the Seven theme.
  */
 
-use Drupal\Component\Utility\Xss;
 use Drupal\Core\Form\FormStateInterface;
 
 /**