how to theme select box drupal for css framework
You need to override theme_select(), which you can do in your theme's template.php file:

function MYTHEME_select($vars) {
  // Your code to produce the markup you need. You can find all pertinent data in $vars.
  // The original form element is at $vars['element'], so the options array is at 
  // $vars['element']['#options'].
}