您可以为所有注解设置描边：

navigation: {
  annotationsOptions: {
    shapeOptions: {
      stroke: 'red'
    }
  }
}

或仅为segment指定annotationsOptions：

navigation: {
  bindings: {
    segment: {
      annotationsOptions: {
        shapeOptions: {
          stroke: 'red'
        }
      }
    }
  }
}

现场演示：https://jsfiddle.net/BlackLabel/14qt9Le0/
API引用：https://api.highcharts.com/highstock/navigation.bindings.segment