borderWidth

Specifies the thickness of the border of the series element being selected with the mouse.

seriesSettings: {
    rangebar: {
        enablePointSelection:true,
        activeSettings: {
            pointSelectedState: {
            enabled: true,  
            borderColor: 'green',   
            borderWidth:3
            }
        }  
    }
}

This setting is demonstrated in the following example.