assignActiveStyle(nodeIds) {

    if (!Array.isArray(nodeIds)) nodeIds = [ nodeIds ];

    const defaultColor = this.visOptions.nodes.color;

    // iterate over selected nodes
    for (let i = nodeIds.length; i--;) {
      const id = nodeIds[i];
      const node = this.graphData.nodesById[id];
      const colorObj = utils.merge({}, defaultColor, node.color);
      this.graphData.nodes.update({
        id: id,
        color: {
          highlight: colorObj,
          hover: colorObj
        }
      });
    }

  }
Assign some styles when the graph element becomes active, i.e.
it is selected or hovered over.

@param {Id|Array<Id>} nodeIds - A single id or an Array of ids.