Update colors of material design PDF viewer.

chrome/browser/resources/pdf/index-material.css

 /* Copyright 2015 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. */
 
 @font-face {
   font-family: 'Roboto';
   font-style: normal;
   font-weight: 400;
   src: local('Roboto Regular'), local('Roboto-Regular'), url(chrome://resources/roboto/roboto.woff2) format('woff2');
   unicode-range: U+0460-052F, U+20B4, U+2DE0-2DFF, U+A640-A69F;
 }
 
 body {
-  background-color: #eee;
+  background-color: #424242;
   font-family: Roboto, 'Helvetica Neue', Helvetica, Arial;
   margin: 0;
 }
 
 viewer-pdf-toolbar {
   position: fixed;
   width: 100%;
   /* TODO(alexandrec): the viewer-pane's paper-dialog is set to z-index: 12
    * internally by the polymer element. Figure out why that is happening. */
   z-index: 1000;
@@ skipping 28 matching lines @@
 
 viewer-error-screen {
   visibility: hidden;
   z-index: 2;
 }
 
 viewer-password-screen {
   visibility: hidden;
   z-index: 2;
 }