MD WebUI: Move some settings CSS to shared_style_css

ui/webui/resources/cr_elements/shared_style_css.html

 <link rel="import" href="chrome://resources/html/polymer.html">
+<link rel="import" href="chrome://resources/cr_elements/shared_vars_css.html">
 
 <!-- Common styles for Material Design WebUI. Included directly in
   settings_shared_css.html. -->
 <dom-module id="cr-shared-style">
   <template>
     <style>
       .action-button {
         background: var(--google-blue-500);
         color: white;
         --paper-button-flat-keyboard-focus: {
           background-color: rgb(58, 117, 215);  /* 88% of --google-blue-500 */
         };
       }
 
       .action-button[disabled] {
         opacity: .25;  /* TODO(dbeam): check this value with bettes. */
       }
 
       .cancel-button {
         --paper-button-flat-keyboard-focus: {
           background-color: rgba(0, 0, 0, .12);
         };
       }
 
       .action-button,
       .cancel-button {
         font-weight: 500;
       }
+
+      [actionable] {
+        @apply(--cr-actionable);
+      }
     </style>
   </template>
 </dom-module>