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

chrome/browser/chromeos/login/lock_window_aura.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/lock_window_aura.h"
 
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "ash/wm/window_animations.h"
+#include "base/command_line.h"
+#include "chrome/common/chrome_switches.h"
 #include "ui/aura/root_window.h"
 #include "ui/aura/window.h"
 
 namespace chromeos {
 
 LockWindow* LockWindow::Create() {
   return new LockWindowAura();
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ skipping 15 matching lines @@
   Init();
 }
 
 LockWindowAura::~LockWindowAura() {
 }
 
 void LockWindowAura::Init() {
   views::Widget::InitParams params(
       views::Widget::InitParams::TYPE_WINDOW_FRAMELESS);
   params.show_state = ui::SHOW_STATE_FULLSCREEN;
+  if (!CommandLine::ForCurrentProcess()->HasSwitch(switches::kDisableNewOobe))
+    params.transparent = true;
   // TODO(oshima): move the lock screen harness to ash.
   params.parent =
       ash::Shell::GetContainer(
           ash::Shell::GetPrimaryRootWindow(),
           ash::internal::kShellWindowId_LockScreenContainer);
   views::Widget::Init(params);
   ash::SetWindowVisibilityAnimationTransition(GetNativeView(),
                                               ash::ANIMATE_NONE);
 }
 
 }  // namespace chromeos