Cell styles created in IronCalc are not properly exported to Excel

[dg-ac]

Cell styles created in IronCalc are not fully exported when saving as .xlsx. While the visual styling is preserved and visible in Excel, the styles are not recognised as named cell styles in Excel, so they cannot be edited or deleted.

Steps to reproduce

1. Open any workbook in IronCalc and create a new cell style.
2. Apply it to one or more cells.
3. Download the workbook as .xlsx and open it in Excel.
4. Open the cell styles panel in Excel and look for the exported style.
5. The styling is visually present on the cells in Excel, but the named style is not exported