Options for the halo appearing around the hovered point in
line-type series as well as outside the hovered slice in pie
charts. By default the halo is filled by the current point or
series color with an opacity of 0.25. The halo can be
disabled by setting the halo option to null.
In styled mode, the halo is styled with the
.highcharts-halo class, with colors inherited from
.highcharts-color-{n}.