Оформление иконки кластера (markerclustererplus library)

cluster.icon.add.js

/**
 * Adding the cluster icon to the dom.
 * @ignore
 */

ClusterIcon.prototype.onAdd = function() {
  this.div_ = document.createElement('DIV');
  if (this.visible_) {
    var pos = this.getPosFromLatLng_(this.center_);
    this.div_.style.cssText = this.createCss(pos);

    var innerHtml;

    if (this.cluster_.markers_.length > 0) {
        innerHtml = "<div><p class='clusterIconText'>" + this.sums_.text + "</p></div>";
    }

    this.div_.innerHTML = innerHtml;
  }

  var panes = this.getPanes();
  panes.overlayMouseTarget.appendChild(this.div_);

  var that = this;
  google.maps.event.addDomListener(this.div_, 'click', function() {
    that.triggerClusterClick();
  });
};

 .clusterIconText
        {
            margin-top:-70px;
            margin-left:-70px; 
            color:#f2f2f2;
        }