RadiusHeatmap3-highlight.css

.radial-heatmap circle.highlight {
  stroke: #FA7F9F;
}

RadiusHeatmap3-highlight.js

{data.map((d, i) => {
  // set the highlight class name if this element is highlighted
  const className = highlight && d[radiusKey] === highlight[radiusKey] ? 'highlight' : '';
  return (
    <circle key={i} className={className} r={r(d[radiusKey])} cx={width / 2}
            cy={-r(0) / 2} strokeWidth={strokeWidth} stroke={color(d[colorKey])}
            onMouseOver={this._handleHover.bind(this, d)}
            onMouseOut={this._handleHover.bind(this, null)}
          />
 );
})}