dynamic stylesheet

background-img.html

<polymer-element name="background-img" attributes="notitle">
  <template>
    <link href="background-img{{num}}.css" rel="stylesheet" />
    <h1>Hello from background-img</h1>
    <content></content>
  </template>

  <script>

    Polymer('background-img', {
      created: function() {
        this.num = 1;
      }
    });

  </script>

</polymer-element>