How to set cells' background?

In the end I changed my mind to use cell background and used fonts. Thanks to answer by foson in SO Creating Excel document with OpenXml sdk 2.0 I managed to add a new Font and a new CellFormat, having preserved the original cell's formatting (i.e. having changed the font color only):

SpreadsheetDocument doc = SpreadsheetDocument.Open("1.xlsx", true);
Sheet sheet = (Sheet)doc.WorkbookPart.Workbook.Sheets.FirstOrDefault();
WorksheetPart worksheetPart = (WorksheetPart)doc.WorkbookPart
                                                .GetPartById(sheet.Id);
Worksheet worksheet = worksheetPart.Worksheet;

WorkbookStylesPart styles = doc.WorkbookPart.WorkbookStylesPart;
Stylesheet stylesheet = styles.Stylesheet;
CellFormats cellformats = stylesheet.CellFormats;
Fonts fonts = stylesheet.Fonts;

UInt32 fontIndex = fonts.Count;
UInt32 formatIndex = cellformats.Count;

Cell cell = GetCell(worksheet, "A", 19);
cell.CellValue = new CellValue(DateTime.Now.ToLongTimeString());
cell.DataType = new EnumValue<CellValues>(CellValues.String);

CellFormat f = (CellFormat)cellformats.ElementAt((int)cell.StyleIndex.Value);

var font = (Font)fonts.ElementAt((int)f.FontId.Value);
var newfont = (Font)font.Clone();
newfont.Color = new Color() { Rgb = new HexBinaryValue("ff0000") };
fonts.Append(newfont);

CellFormat newformat = (CellFormat)f.Clone();
newformat.FontId = fontIndex;
cellformats.Append(newformat);

stylesheet.Save();

cell.StyleIndex = formatIndex;
doc.Close();