How to format just specific cells?

Hi guys,

I know how to format rows and columns but how can I format just specific cells?

<table>
  <tr bgcolor="#FF0000">
    <th>Month</th>
    <th>Savings</th>
  </tr>
  <tr>
    <td>January</td>
    <td>$100</td>
  </tr>
  <tr>
    <td>January</td>
    <td>**$200**</td>
  </tr>
</table>

So for example how to just use a specific css class on values over 150$?