How to keep the background color when you edit the excel sheet by POI

Question

I code in C#. I use NPOI(version 2.5.5). But I link this OP to Apache POI as well because the solution for this would be the same for both modules.

I have this excel file whose extention is .xlsx:

I editted the Excel sheet via NPOI.

code:

private void button_editByNpoi_Click(object sender, EventArgs e)
        {
            string filePath = destFileName;

            IWorkbook workbook;
            using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read))
            {
                workbook = new XSSFWorkbook(fs);
            }

            /*
            //1枚目のシートを取得
            ISheet sheet = workbook.GetSheetAt(0);

            //セルを取得
            IRow row = sheet.GetRow(1);
            ICell cell = row.GetCell(1);

            //セルに書き込み
            //cell.SetCellValue("a");
            */

            using (FileStream fs = new FileStream(filePath, FileMode.Create, FileAccess.Write))
            {
                //上書き保存する。
                workbook.Write(fs);
            }
        }

As the result, I got this excel sheet:

the light gray color has gone after editting. How can I keep the color?