frm_field_div_classes

This example will allow you to add a CSS class called 'class_name' to the div surrounding your field.

add_filter('frm_field_div_classes', 'add_div_class', 10, 2);
function add_div_class($classes, $field){
  $space = ' ';
  if($field['id'] == 3137){ //change 3137 to the ID of your field
        $classes .= $space . 'class_name'; //change class_name to your CSS class
  }
  return $classes;
}