<style>

  core-header-panel {

    background: red;

  }

  #content {

    background: yellow;

    min-height: calc(100% - 32px /*core-toolbar*/ - 32px /*footer*/);

  }

  footer {

    background: green;

    height: 32px;

  }

</style>

<core-header-panel>

  <core-toolbar>

    Header

  </core-toolbar>

  <div id='content'> 

    ...

  </div>

  <footer>

    FOOTER

  </footer>

</core-header-panel>