I replaced 188.8.131.52 to latest version and I found modifing defaultstyle is not applied as general style.
please try following code and open output file.
Workbook workbook = new Workbook();
Style style = workbook.DefaultStyle;
style.Font.Name = "Tahoma";
style.Font.Size = 11;
style.VerticalAlignment = TextAlignmentType.Top;
workbook.DefaultStyle = style;
Font "Arial 10"
Do I need to try another new way to set this property?