MD Settings: make the background color of the collapsed nav drawer white

chrome/browser/resources/settings/settings_menu/settings_menu.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icons/av-icons.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icons/device-icons.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icons/hardware-icons.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icons/image-icons.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icons/iron-icons.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icons/social-icons.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-menu/paper-menu.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-menu/paper-submenu.html">
@@ skipping 14 matching lines @@
       .iron-selected:not(.menu-trigger) > iron-icon {
         fill: var(--google-blue-500);
       }
 
       .menu-trigger span {
         -webkit-margin-end: 16px;
         flex: 1;
       }
 
       paper-menu {
-        background-color: var(--settings-background-color);
+        background: none;  /* Overrides <paper-menu> default. */
         color: var(--settings-nav-grey);
         font-size: 13px;
         --paper-menu-selected-item: {
           font-weight: 500;
         };
         --paper-menu-focused-item-after: {
           background: none;
         };
       }
 
@@ skipping 120 matching lines @@
       </paper-submenu>
       <paper-submenu on-tap="openPage_">
         <div class="menu-trigger">
           <span i18n-content="aboutProgram"></span>
         </div>
       </paper-submenu>
     </paper-menu>
   </template>
   <script src="settings_menu.js"></script>
 </dom-module>