I found that there is a property for Bollinger Band thickness (StrokeThickness):
BollingerBandsIndicator indicator =
indicator.Period = 20;
indicator.StandardDeviations = 2;
indicator.Stroke = Brushes.Red;
indicator.LowerBandStroke = Brushes.Red;
indicator.StrokeThickness = 1;
However, when the indicator displays, only the upper band thickness will be effective for changing the StrokeThickness. I cannot find how to change the thickness of the lower band.