[cros] Add support for transparent background in sign in screen.

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

 /* Copyright (c) 2012 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 is the stylesheet used by the Out of the box experience (OOBE) flow,
  * sign in and lock screens.
  */
 
 html,
 body {
   height: 100%;
   width: 100%;
 }
 
 body {
-  background: #fefefe;
+  background-color: #fefefe;
   font-size: 14px;
   margin: 0;
   overflow: hidden;
   padding: 0;
 }
 
+html[oobe=new]:not([screen=lock]) body {
+  background-color: transparent;
+}
+
 progress {
   border: solid 1px #bbb;
   padding: 1px;
 }
 
 progress::-webkit-progress-bar {
   background: white;
 }
 
 progress::-webkit-progress-value {
@@ skipping 807 matching lines @@
 }
 
 #notification-area a {
   color: rgb(37, 79, 155);
 }
 
 #notification-area .butter-bar {
   margin: 0 auto;
   max-width: 850px;
 }