How can I define fieldset border color?

I want to set border color of field set. I am using class but this is not working properly because i want to remove fieldset default border color. so how can I use fieldset border color.

<fieldset class="field_set">
    <legend>box</legend>
     <table width="100%" border="0" cellspacing="0" cellpadding="0">
           <tr>
               <td>&nbsp;</td>
           </tr>
      </table>
</fieldset>

css

.field_set{
 border-color:#F00;
}