polymer/test/unit/custom-style-late-import.html

<script>
  Polymer({
    is: 'x-input',
    extends : 'input'
  });
</script>
<style is="custom-style">
  input[is=x-input] {
    border : 4px solid red;
    @apply (--cs-blue);
  }
</style>
<style is="custom-style">
  :root {
    --cs-blue: {
      border : 8px solid blue;
    };
  }
</style>