2nd display should show the same background as login/lock screen:

chrome/browser/chromeos/login/webui_login_view.cc

 // 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.
 
 #include "chrome/browser/chromeos/login/webui_login_view.h"
 
 #include "ash/shell.h"
 #include "ash/system/tray/system_tray.h"
 #include "base/bind.h"
 #include "base/callback.h"
@@ skipping 185 matching lines @@
 }
 
 void WebUILoginView::UpdateWindowType() {
 }
 
 void WebUILoginView::LoadURL(const GURL & url) {
   webui_login_->LoadInitialURL(url);
   webui_login_->RequestFocus();
 
   CommandLine* command_line = CommandLine::ForCurrentProcess();
-  // Only enable transparency on sign in screen, not on lock screen.
-  if (BaseLoginDisplayHost::default_host() &&
-      !command_line->HasSwitch(switches::kDisableNewOobe)) {
+  if (!command_line->HasSwitch(switches::kDisableNewOobe)) {
     // TODO(nkostylev): Use WebContentsObserver::RenderViewCreated to track
     // when RenderView is created.
     // Use a background with transparency to trigger transparency in Webkit.
     SkBitmap background;
     background.setConfig(SkBitmap::kARGB_8888_Config, 1, 1);
     background.allocPixels();
     background.eraseARGB(0x00, 0x00, 0x00, 0x00);
     content::RenderViewHost* host =
         tab_contents_->web_contents()->GetRenderViewHost();
     host->GetView()->SetBackground(background);
@@ skipping 139 matching lines @@
   aura::Env::GetInstance()->set_render_white_bg(true);
 }
 
 void WebUILoginView::ReturnFocus(bool reverse) {
   // Return the focus to the web contents.
   webui_login_->web_contents()->FocusThroughTabTraversal(reverse);
   GetWidget()->Activate();
 }
 
 }  // namespace chromeos