How can I add  to all checkboxes?
Yes, there is a theme hook that will let you alter every instance of a checkbox. The two theme hooks you'll want to look at are:


theme_checkbox() - https://api.drupal.org/api/drupal/includes!form.inc/function/theme_checkbox/7
theme_checkboxes() - https://api.drupal.org/api/drupal/includes!form.inc/function/theme_checkboxes/7


Sample Code

Here's some sample code, available in the API doc links I provided above.

function theme_checkbox($variables) {
  $element = $variables['element'];
  $element['#attributes']['type'] = 'checkbox';
  element_set_attributes($element, array('id', 'name','#return_value' =&gt; 'value'));

  // Unchecked checkbox has #value of integer 0.
  if (!empty($element['#checked'])) {
    $element['#attributes']['checked'] = 'checked';
  }
  _form_set_class($element, array('form-checkbox'));

  return '&lt;input' . drupal_attributes($element['#attributes']) . ' /&gt;';
}


You can override the markup returned by implementing this theme hook.