Fix white flash when opening incognito NTP

chrome/browser/resources/ntp4/md_incognito_tab.css

 /* Copyright 2017 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. */
 
-html.md {
-  background-color: #303030;
-}
-
 body {
   -webkit-font-smoothing: antialiased;
   font-size: 100%;
   margin: 0;
 }
 
 /** Typography -------------------------------------------------------------- */
 
 .content {
+  /* This is identical to the default background color. It's necessary to set it
+     for the case when a theme with a background image is installed. */
   background-color: #303030;
   color: rgba(255, 255, 255, 0.7);
   font-size: calc(100% - 2px);
   line-height: calc(100% + 6px);
   min-width: 240px;
 }
 
 h1 {
   color: rgba(255, 255, 255, 0.8);
   font-size: calc(100% + 8px);
@@ skipping 266 matching lines @@
 }
 
 /* Very narrow screens. */
 @media (max-width: 240px) {
   .content {
     min-width: 192px;
     padding-left: 24px;
     padding-right: 24px;
   }
 }