[MD settings] change color values to --paper-variable colors

chrome/browser/resources/settings/settings_ui/settings_ui.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. */
 
 :host {
   @apply(--layout-vertical);
   @apply(--layout-fit);
   @apply(--paper-font-body1);
   -webkit-user-select: none;
-  background-color: #eceff1;
+  background-color: var(--paper-blue-grey-50);
 }
 
 paper-toolbar {
   @apply(--shadow-elevation-4dp);
   background-color: rgb(52, 73, 94);
   height: 56px;
   position: relative;
   z-index: 10;
 }
 
 paper-drawer-panel {
   @apply(--layout-center);
   --paper-drawer-panel-left-drawer-container: {
     background-color: rgb(236, 239, 241);
     margin-top: 8px;
   };
   position: relative;
 }
 
 settings-main paper-icon-button {
   z-index: 10;
 }