ChromeOS OOBE: Implement network selection screen "Back" button.

chrome/browser/resources/chromeos/login/oobe_screen.css

 /* Copyright 2013 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  *
  * This contains common styling for all the OOBE screens. */
 
 :root {
   --oobe-roboto-medium: 500 12px Roboto;
+  --oobe-roboto-bold: 700 12px Roboto;
 }
 
 .step {
   box-sizing: border-box;
   position: absolute;
 }
 
 .step.animated:not(.faded) {
   -webkit-transition: -webkit-transform 200ms ease-in-out,
                       opacity 200ms ease-in-out,
@@ skipping 150 matching lines @@
 
 html[highlight=strong] #security-info a:focus,
 html[highlight=strong] #eula a:focus,
 html[highlight=strong] .step-extra-controls a:focus,
 html[highlight=strong] .menu-control:focus,
 html[highlight=strong] input[type='button']:focus,
 html[highlight=strong] button:focus {
   box-shadow: 0 0 23px rgb(77, 144, 254) !important;
 }