How to change the style of a Select box' optgroup label

I have a simple select box with an optiongroup in my application.

<select>
   <optgroup label="Swedish Cars">
     <option value="volvo">Volvo</option>
     <option value="saab">Saab</option>
 </optgroup>
  ----
  ----
  ----
</select>

When it gets displayed in browser, the option group label is displayed with bold and italic; I want it to be displayed without any of those styles.