vaadin-theme-property-mixin.html

<script>
  /**
   * @namespace Vaadin
   */
  window.Vaadin = window.Vaadin || {};

  /**
   * @polymerMixin
   * @memberof Vaadin
   */
  Vaadin.ThemePropertyMixin = superClass => class VaadinThemePropertyMixin extends superClass {
    static get properties() {
      return {
        /**
         * Helper property with theme attribute value facilitating propagation
         * in shadow DOM.
         *
         * Enables the component implementation to propagate the `theme`
         * attribute value to the subcomponents in Shadow DOM by binding
         * the subcomponent’s "theme" attribute to the `theme` property of
         * the host.
         *
         * **NOTE:** Extending the mixin only provides the property for binding,
         * and does not make the propagation alone.
         *
         * See [Theme Attribute and Subcomponents](https://github.com/vaadin/vaadin-themable-mixin/wiki/5.-Theme-Attribute-and-Subcomponents).
         * page for more information.
         *
         * @protected
         */
        theme: {
          type: String,
          readOnly: true
        }
      };
    }

    /** @protected */
    attributeChangedCallback(name, oldValue, newValue) {
      super.attributeChangedCallback(name, oldValue, newValue);

      if (name === 'theme') {
        this._setTheme(newValue);
      }
    }
  };
</script>