Are you interested in adding the workbook styles specifically or are you just looking for a way to set the style of cells? If you want to add workbook styles, you can do something like this:

Workbook workbook = new Workbook();
Worksheet worksheet = workbook.Worksheets.Add("Sheet1");
 
IWorksheetCellFormat style = workbook.CreateNewWorksheetCellFormat();
style.Font.Name = "Verdana";
style.Font.Height = 240;
style.Font.Color = System.Drawing.Color.Red;
style.FillPatternForegroundColor = System.Drawing.Color.Gray;
style.FillPattern = FillPatternStyle.Solid;
workbook.Styles.AddUserDefinedStyle(style, "NewStyle");
 
WorksheetCell cell = worksheet.GetCell("A1");
cell.Value = "Test";
cell.CellFormat.SetFormatting(style);
 
workbook.Save("Styles.xls");

 But if you just want to change the style of a cell, you can do this:

Workbook workbook = new Workbook();
Worksheet worksheet = workbook.Worksheets.Add("Sheet1");
			
WorksheetCell cell = worksheet.GetCell("A1");
cell.Value = "Test";
cell.CellFormat.Font.Name = "Verdana";
cell.CellFormat.Font.Height = 240;
cell.CellFormat.Font.Color = System.Drawing.Color.Red;
cell.CellFormat.FillPatternForegroundColor = System.Drawing.Color.Gray;
cell.CellFormat.FillPattern = FillPatternStyle.Solid;
 
workbook.Save("Styles.xls");