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

chrome/browser/chromeos/login/webui_login_view.cc

Index: chrome/browser/chromeos/login/webui_login_view.cc
diff --git a/chrome/browser/chromeos/login/webui_login_view.cc b/chrome/browser/chromeos/login/webui_login_view.cc
index b770eff66f2b18609e15e8688d453e8e5722e612..2803e56a069fb33991eaa205fa0b2c8c43975fbd 100644
--- a/chrome/browser/chromeos/login/webui_login_view.cc
+++ b/chrome/browser/chromeos/login/webui_login_view.cc
@@ -203,9 +203,7 @@ void WebUILoginView::LoadURL(const GURL & 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.