       --mixin: {
          color: blue;
        };
      }
    </style>
  </custom-style>

  <script type="module">
'use strict';
class UnknownHost extends HTMLElement {
  constructor() {
    super();
    this.attachShadow({mode: 'open'});
  }
}
customElements.define('unknown-host', UnknownHost);
</script>

  <dom-module id="x-foo">
    <template>
    <style>
      :host {
        border: var(--border);
        display: block;
        @apply --mixin;
      }
    </style>
      x-foo
    </template>
    <sc