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

chrome/browser/chromeos/login/lock_window_aura.cc

Index: chrome/browser/chromeos/login/lock_window_aura.cc
diff --git a/chrome/browser/chromeos/login/lock_window_aura.cc b/chrome/browser/chromeos/login/lock_window_aura.cc
index 90b4115a8bd49cc8c073e234b32ab8476bb65cdd..65be222d30b6234461d8896df78430124504ef7a 100644
--- a/chrome/browser/chromeos/login/lock_window_aura.cc
+++ b/chrome/browser/chromeos/login/lock_window_aura.cc
@@ -7,6 +7,8 @@
 #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"
 
@@ -42,6 +44,8 @@ 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(