borderWidth

Sets the border width of the selected bars:

seriesSettings: {
    polarbar: {
        enablePointSelection: true,
        activeSettings: {            
            pointSelectedState: {                
                color: 'green',
                borderColor: 'green',
                borderWidth: 14,
                borderRadius: 5
            }
        }
    }
}

This setting is demonstrated in the following example.