css(property, value, priority) {
    var content = this.content || this.document.body;

    if (value) {
      content.style.setProperty(property, value, priority ? "important" : "");
    } else {
      content.style.removeProperty(property);
    }

    return this.window.getComputedStyle(content)[property];
  }
Set Css styles on the contents element (typically Body)
@param {string} property
@param {string} value
@param {boolean} [priority] set as "important"