constructor(rendition) {
    this.rendition = rendition;
    this._themes = {
      "default": {
        "rules": {},
        "url": "",
        "serialized": ""
      }
    };
    this._overrides = {};
    this._current = "default";
    this._injected = [];
    this.rendition.hooks.content.register(this.inject.bind(this));
    this.rendition.hooks.content.register(this.overrides.bind(this));
  }
Themes to apply to displayed content
@class
@param {Rendition} rendition