MD Downloads: fix some a11y issues

chrome/browser/resources/md_downloads/manager.html

 <link rel="import" href="chrome://resources/html/cr.html">
 <link rel="import" href="chrome://resources/html/cr/ui.html">
 <link rel="import" href="chrome://resources/html/cr/ui/command.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/util.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-list/iron-list.html">
 <link rel="import" href="chrome://downloads/action_service.html">
 <link rel="import" href="chrome://downloads/constants.html">
 <link rel="import" href="chrome://downloads/i18n_setup.html">
 <link rel="import" href="chrome://downloads/item.html">
@@ skipping 55 matching lines @@
         flex: 1;
       }
 
       :host([loading]) #no-downloads,
       :host([loading]) #downloads-list {
         display: none;
       }
 
       #no-downloads {
         align-items: center;
-        color: #b4b4b4;
+        color: #6e6e6e;
         display: flex;
         font-size: 123.1%;
         font-weight: 500;
         justify-content: center;
         /* To avoid overlapping with the header, we need this min-height
          * until bug 596743 is fixed. */
         min-height: min-content;
       }
 
       #no-downloads .illustration {
@@ skipping 16 matching lines @@
     </iron-list>
     <div id="no-downloads" hidden="[[hasDownloads_]]">
       <div>
         <div class="illustration"></div>
         <span>[[noDownloadsText_(inSearchMode_)]]</span>
       </div>
     </div>
   </template>
   <script src="chrome://downloads/manager.js"></script>
 </dom-module>