When defaultCellStyle is used the configured font is applied to the cells in the Spreadsheet. However, the font is not applied when adding new rows.
The configured font in the defaultCellStyle is not taken into account. The font in the newly inserted row is Arial 12, while in the example it should be 'Courier New' 18.
The font configured by using the defaultCellStyle option should be applied to all cells in the Spreadsheet, even if inserted after initialization.