default(theme) {
    if (!theme) {
      return;
    }

    if (typeof theme === "string") {
      return this.registerUrl("default", theme);
    }

    if (typeof theme === "object") {
      return this.registerRules("default", theme);
    }
  }
Add a default theme to be used by a rendition
@param {object | string} theme
@example themes.register("http://example.com/default.css")
@example themes.register({ "body": { "color": "purple"}})