Make the hint text black.

ui/keyboard/resources/elements/kb-key.html

 <!--
   -- Copyright 2013 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.
   -->
 
 <polymer-element name="kb-key" extends="kb-key-base"
     attributes="image imageSize keyCode keyName shiftModifier weight">
   <template>
     <style>
@@ skipping 23 matching lines @@
         left: 0;
         margin: auto;
         padding-left: 0.7em;
         padding-right: 0.7em;
         position: absolute;
         right: 0;
         top: 0;
       }
 
       ::part(hint) {
-        color: #7c7c7c;
+        color: #313131;
         font-size: 70%;
         position: absolute;
         right: 7%;
         top: 5%;
       }
 
       :host[invert] ::part(key) {
-        color: #7c7c7c;
+        color: #313131;
       }
 
       :host[invert] ::part(hint) {
         color: #ffffff;
       }
     </style>
     <div id="key" part="key">
       <content></content>
     </div>
     <div part="hint">{{hintText}}</div>
@@ skipping 136 matching lines @@
       up: function(event) {
         if (this.longPressTimer) {
           clearTimeout(this.longPressTimer);
           hideKeyboard();
           this.longPressTimer = undefined;
         }
       }
     });
   </script>
 </polymer-element>