need wp_dropdown_categories hack: styling for different cat levels

Is there a way to define my own styles for top-level (or any other level) categories in wp_dropdown_categories? The current output does not assign css classes to the <option> tags:

<option value=”14″>1st Japanese Release</option>
<option value=”25″>   Helic</option>
<option value=”26″>   Zenebas</option>
<option value=”27″>   Guylos/Etc</option>

I imagine the answer lies somewhere within preg_replace(), but I still can’t wrap my mind around regular expressions :(.

thanks in advance,
rcm