code: function toggle(progress) {
    var opts = this.options;
    var element = this.element;
    var times = Object.keys(opts);
  
    times.forEach(function(time) {
      var css = opts[time];
  
      if (progress > time) {
        element.classList.add(css);
      } else {
        element.classList.remove(css);
      }
    });
  }
docs: Toggle a class on or off.
  @param {Float} progress: Current progress data of the scene, between 0 and 1.
  @this {Object}
  @return {void}