Thanks for your posting and using Aspose.Cells.
You can change the default font of the workbook using the following code. The code changes the default font to Arial Narrow and the font size to 15.
I have attached the output xlsx file generated by it for your reference.
Workbook workbook = new Workbook();
Style style = workbook.CreateStyle();
style.Font.Name = “Arial Narrow”;
style.Font.Size = 15;
workbook.DefaultStyle = style;