code: public void addCellStyle (@Nonnull final ExcelStyle aExcelStyle)
    {
      ValueEnforcer.notNull (aExcelStyle, "ExcelStyle");
      if (m_aLastCell == null)
        throw new IllegalStateException ("No cell present for current row!");
  
      CellStyle aCellStyle = m_aStyleCache.getCellStyle (aExcelStyle);
      if (aCellStyle == null)
      {
        aCellStyle = m_aWB.createCellStyle ();
        aExcelStyle.fillCellStyle (m_aWB, aCellStyle, m_aCreationHelper);
        m_aStyleCache.addCellStyle (aExcelStyle, aCellStyle);
        m_nCreatedCellStyles++;
      }
      m_aLastCell.setCellStyle (aCellStyle);
    }
docs: Set the cell style of the last added cell
  
  @param aExcelStyle
  The style to be set.