Improve the appearance of <sky-checkbox>

sky/framework/sky-box/sky-box.sky

 <!--
 // Copyright 2014 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.
 -->
 <import src="/sky/framework/sky-element/sky-element.sky" as="SkyElement" />
 
 <sky-element name="sky-box">
 <template>
   <style>
     :host {
       display: flex;
       flex-direction: column;
       border-radius: 4px;
       border: 1px solid gray;
       margin: 10px;
     }
-    #title {
+    .title {
       text-align: center;
       font-size: 10px;
+      padding: 8px 8px 4px 8px;
+    }
+    .content {
+      padding: 4px 8px 8px 8px;
     }
     div {
-      flex: 1 100%;
+      flex: 1;
     }
   </style>
-  <div id='title'>{{ title }}</div>
-  <div><content></content></div>
+  <div class="title">{{ title }}</div>
+  <div class="content"><content></content></div>
 </template>
 <script>
 module.exports = class extends SkyElement {
 }.register();
 </script>
 </sky-element>