Created attachment 27249 [details]
extraced xlsxs 'a' & 'b'

for HSSFRow there is setRowStyle method, XSSFRow lacks this method.

attached you'll find extracted xlsx a & b, the differing files are xl/worksheets/sheet1.xml resp.. in both files there is a '1' in cell A1, which has a certain format (bottom border black thin). in file 'b' the 1st row additionally has the same format set as row format.

the differing part is:

<row r="1" spans="1:1">
vs.
<row r="1" spans="1:1" s="1" customFormat="1">

so i guess the code from XSSFCell should work for XSSFRow (+ customFormat), so it should be something like:

    public void setCellStyle(CellStyle style) {
        if(style == null) {
            if(_row.isSetS()) { // + maybe something like: "|| _row.isSetCustomFormat()"
                _row.unsetS();
                _row.unsetCustomFormat();
            }
        } else {
            XSSFCellStyle xStyle = (XSSFCellStyle)style;
            xStyle.verifyBelongsToStylesSource(_stylesSource);

            long idx = _stylesSource.putStyle(xStyle);
            _row.setS(idx);
            _row.setCustomFormat(true);
        }
    }

the _stylesSource of XSSFRow could be handled exactly like for XSSFCell (as it's only a pointer to row.getSheet().getWorkbook().getStylesSource()).