MD Settings: Tweak action menu styling to match the mocks.

ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 
 <dom-module id="cr-action-menu">
   <template>
     <style>
       :host {
         background-color: white;
         border: none;
         box-shadow: 0 2px 6px var(--paper-grey-500);
         margin: 0;
         outline: none;
-        padding: 0;
+        padding: 8px 0;
       }
 
       :host::backdrop {
         background-color: transparent;
       }
+
+      :host ::content .dropdown-item:focus {
+        background-color: var(--paper-grey-300);
+        outline: none;
+      }
     </style>
     <content select=".dropdown-item,hr"></content>
   </template>
   <script src="cr_action_menu.js"></script>
 </dom-module>