docs: add executeScript demo

docs/themes.md

@@ -12,3 +12,37 @@ There are currently three themes available. Copy [Vue](//vuejs.org) and [buble](
 !> This compressed files in `/lib/themes/`.
 
 If you have any ideas or would like to develop new theme, welcome submit [PR](https://github.com/QingWei-Li/docsify/pulls).
+
+#### Click to preview
+
+
+<div class="demo-theme-preview">
+  <a data-theme="vue">vue.css</a>
+  <a data-theme="buble">buble.css</a>
+  <a data-theme="dark">dark.css</a>
+</div>
+
+
+<style>
+  .demo-theme-preview a {
+    padding-right: 10px;
+  }
+
+  .demo-theme-preview a:hover {
+    text-decoration: underline;
+    cursor: pointer;
+  }
+</style>
+
+<script>
+  var preview = Docsify.dom.find('.demo-theme-preview');
+  var themes = Docsify.dom.findAll('[rel="stylesheet"]');
+
+  preview.onclick = function (e) {
+    var title = e.target.getAttribute('data-theme')
+
+    themes.forEach(function (theme) {
+      theme.disabled = theme.title !== title
+    });
+  };
+</script>