borderRadius

Specifies the corners smoothing value applicable both for the border of the series element being pointed to with the mouse and the element itself.

seriesSettings: {
    rangebar: {
        activeSettings: {
            pointHoveredState: {
             enabled: true,  
             borderColor: 'green',   
             borderWidth:3,
             borderRadius: 5
            }
        }  
    }
}

This setting is demonstrated in the following example.