DocRaptor supports many CSS style attributes you're already familiar with, plus a few Excel file specific attributes. A complete list of supported attributes is available here. Apply these attributes through inline CSS styles, like this:
<table name="Example Worksheet">
<tr style="font-size: 16px">
<td colspan="2">Big Row</td>
</tr>
<tr>
<td style="color: blue">Blue Cell</td>
<td style="-xls-format:currency_dollar">1000</td>
</tr>
</table>