Background color field is not working in 31-03-2009 version

```
What steps will reproduce the problem?
1. Workbook workbook = new Workbook();
Worksheet worksheet = new Worksheet("First Sheet");
worksheet.Cells[0, 1] = new Cell("teste");
worksheet.Cells.ColumnWidth[0, 1] = 3000;
worksheet.Cells[0, 1].Style = new CellStyle();
worksheet.Cells[0, 1].Style.BackColor = System.Drawing.Color.Gold;
workbook.Worksheets.Add(worksheet);
workbook.Save(file);
2.
3.

What is the expected output? What do you see instead?
I expected to change the cell background color, but it doesn't work.

What version of the product are you using? On what operating system?
ExcelLibrary_20090331.zip. There's another issue that says that in a new
version (up from 12-02-2009) this issue is solved, I tried in 31-03-2009
version, but it's not working.

Please provide any additional information below.

```

Original issue reported on code.google.com by `andrefix...@gmail.com` on 29 Sep 2009 at 5:07

Reproduce the issue with the shown Workbook, Worksheet, CellStyle.BackColor, and Save sequence using ExcelLibrary_20090331.zip. Trace how BackColor is written during workbook saving; done when the resulting spreadsheet displays the cell with a gold background.

Written by the indexing model from the issue text.