ChromeOS OOBE: make "System Security Settings" popup dialog backgroung gray.

chrome/browser/resources/chromeos/login/oobe_popup_overlay.css

 /* Copyright 2014 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. */
 
 .popup-overlay {
   -webkit-transition: 250ms opacity;
   align-items: center;
   background: rgba(255, 255, 255, .75);
   bottom: 0;
   display: flex;
   justify-content: center;
   left: 0;
   padding: 20px;
   position: fixed;
   right: 0;
   top: 0;
   z-index: 5;
 }
 
+#popup-overlay[md-mode] {
+  background: rgba(0, 0, 0, .5);
+}
+
 .oobe-popup {
   background: white;
   border: 1px solid rgb(188, 193, 208);
   border-radius: 2px;
   box-shadow: 0 5px 80px #505050;
   min-height: 250px;
   position: relative;
   width: 460px;
   z-index: 10;
 }
 
 .oobe-popup h1 {
   -webkit-padding-end: 24px;
   -webkit-user-select: none;
   background: linear-gradient(white, #F8F8F8);
   color: rgb(83, 99, 125);
   font-size: 105%;
   font-weight: bold;
   margin: 0;
   padding: 10px 15px 8px 15px;
   text-shadow: white 0 1px 2px;
 }
 
 .popup-content-area {
   padding: 20px 20px 60px 20px;
 }