function clusterMemberStyle(clusterMember) {
  return new Style({
    geometry: clusterMember.getGeometry(),
    image: clusterMember.get('LEISTUNG') > 5 ? darkIcon : lightIcon,
  });
}
Single feature style, users for clusters with 1 feature and cluster circles.
@param {Feature} clusterMember A feature from a cluster.
@return {Style} An icon style for the cluster member's location.