title: Change background url dynamically in paper-scroll-header-panel
body: <p>So I have this style that when the header condenses it change its background into red and when the header is at its full size it display a backgroun</p>
  
  <pre><code>paper-scroll-header-panel {
      position: absolute;
      top: 0;
      left: 0;
      right: 0;
      bottom: 0;
      background-color: var(--paper-grey-100);
  
      --paper-scroll-header-panel-full-header: {
        background-image: url(images/bg1.jpg);
      }
  
      --paper-scroll-header-panel-condensed-header: {
        background-color: var(--paper-red-700);
      }
    }
  </code></pre>
  
  <p>The problem is I load the image background url dynamically.
  What is the way to set '--paper-scroll-header-panel-condensed-header' background-image using javascript?</p>