2nd display should show the same background as login/lock 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 {
@@ skipping 10 matching lines @@
 
 html[oobe=old] body {
   background-color: #fefefe;
 }
 
 html[oobe=new]:not([screen=lock]) body {
   background-color: transparent;
 }
 
 html[oobe=new][screen=lock] body {
-  -webkit-background-size: 100% 100%;
+  background-color: transparent;
 }
 
 progress {
   border: solid 1px #bbb;
   padding: 1px;
 }
 
 progress::-webkit-progress-bar {
   background: white;
 }
@@ skipping 1225 matching lines @@
   width: 100%;
 }
 
 .background-initial {
   opacity: 0;
 }
 
 .background-final {
   opacity: 1;
 }