How can I add a class to a label?
This post on StackOverflow says that the Drupal Form API does not support adding classes to labels, but it does contain a decent workaround:


  Instead, you could target it using the wrapper class for the field and label:

.form-item-field-foo label {
  /* CSS here */
}



The API documentation for theme_form_element_label() shows why labels aren't possible (at least not out-of-the-box): the attributes array is initialized in the theme function and populated with just two optional classes ('option' and 'element-invisible').

So, is it completely impossible? In fact, I think it can be done, but I must add that I haven't tried this myself. You could try to:


Add a new #label_attributes property to some (or all) form elements using hook_element_info_alter() in a custom module;
Override theme_form_element_label() and make it merge the $attributes array with the value of $element['#label_attributes'].


If you're going to give this a try, please let us know if it worked.