title: How to use js to query shadow dom element in Polymer?
body: <p>I know how to query shadow dom element in <code>&lt;style&gt;</code> tag,but i want to use data-bind dynamically change the style,data-bind can not be applied in <code>&lt;style&gt;</code> in Polymer,so i should make it happen in js.For example,i use <code>core-scroll-header-panel</code>  component, i can query its background style using:</p>
  
  <pre><code>&lt;style&gt;
      core-scroll-header-panel::shadow #headerBg {
            background: #5cebca;
          }
  
  &lt;/style&gt;
  </code></pre>
  
  <p>but how can implement it in js?</p>