If you want to export a document with modified Normal style, you have to modify it and then set it to at least one cell in order to be exported. This maybe is caused by the fact that if the style is not used it is not exported.

Workaround: Export single cell in the row with this style applied to it:

using (ICellExporter cell = row.CreateCellExporter())
{
    cell.SetFormat(new SpreadCellFormat()
    {
          CellStyle = normalStyle
     });
}