If you look @ the source, you’ll find this:

<select name="RightBodySectionSearchBoxID:SBScopesDDL" id="RightBodySectionSearchBoxID_SBScopesDDL" class="ms-sbdropdown ms-descriptiontext" Title="Select a scope">

Maybe you can add an alternate CSS and overload one of the two styles or just modify the site definition? Careful overloading the styles… I’d search  the rest of the site definition to make sure you aren’t making changes elsewhere in your site definition inadvertently.