How can I override default theme_fieldset() function?

Question

I am trying to override the theme_fieldset function in a custom node edit form where the fieldset is added dynamically.

Here is what my code looks like:

function cms_content_edit(){
  $node = node_load(3089);
  module_load_include('inc', 'node', 'node.pages');
  $form = drupal_get_form('test_node_form', $node);
  $form['block1'] = array(
    '#type' => 'fieldset',
    '#title' => t('Block 1'),
    '#weight' => -50,
    '#collapsible' => true,
    '#collapsed' => true,
    '#attributes' => array('class' => array('portlet box blue-hoki')),
    '#theme' => 'fieldset_custom3',
  );
  $form['block1']['title'] = $form['title'];
  unset($form['title']);
  $form['block1']['body'] = $form['body'];
  unset($form['body']);
  $form['block1']['field_fichier'] = $form['field_fichier'];
  unset($form['field_fichier']);
  $output = drupal_render($form);
  return $output;
}

The custom function is defined this way in my .module file:

function theme_fieldset_custom3($variables) {
  $element = $variables['element'];
  element_set_attributes($element, array('id'));
  _form_set_class($element, array('form-wrapper'));

  $output = '<fieldset' . drupal_attributes($element['#attributes']) . '>';
  if (!empty($element['#title'])) {
    // Always wrap fieldset legends in a SPAN for CSS positioning.
    $output .= '<div class="portlet-title">' . $element['#title'] . '</div>';
  }
  $output .= '<div class="fieldset-wrapper">';
  if (!empty($element['#description'])) {
    $output .= '<div class="fieldset-description">' . $element['#description'] . '</div>';
  }
  $output .= $element['#children'];
  if (isset($element['#value'])) {
    $output .= $element['#value'];
  }
  $output .= '</div>';
  $output .= "</fieldset>\n";
  return $output;
}

Unfortunately my custom function is not called at all.

Did I do something wrong?

******** EDIT *********

Thanks to @kiamlaluno, after I added my custom function to hook_theme(), my custom function was actually loaded.

function mymodule_theme() {
  return array(
    'fieldset_custom3' => array(
      'render element' => 'element',
    ),
  );
}

The problem now is that the .fieldset-wrapper div is empty. In my custom function, the $element['#children'] array is empty...

My fieldset is created dynamically, like that (in hook_form_alter()):

if(isset($form['#node_edit_form']) && $form['#node_edit_form']==TRUE && $form['nid']['#value']!=''){

       $form['block1'] = array(
          '#type' => 'fieldset',
          '#title' => t('Block 1'),
          '#weight' => -50,
          '#collapsible' => false,
          '#collapsed' => false,
        );
        $form['block1']['title'] = $form['title'];
        unset($form['title']);
        $form['block1']['body'] = $form['body'];
        unset($form['body']);
        $form['block1']['field_fichier'] = $form['field_fichier'];
        unset($form['field_fichier']);
        $form['block1']['#theme'] = 'fieldset_custom3';
        // $form['actions']['submit']['#submit']['0'] = 'node_form_submit_custom';
    }

I also tried this way (this is a custom page):

function cms_content_edit(){
  $node = node_load(3089);
  module_load_include('inc', 'node', 'node.pages');
  $form = drupal_get_form('test_node_form', $node);
  $form['block1'] = array(
    '#type' => 'fieldset',
    '#title' => t('Block 1'),
    '#weight' => -50,
    '#collapsible' => true,
    '#collapsed' => true,
    '#attributes' => array('class' => array('portlet box blue-hoki')),
    '#theme' => 'fieldset_custom3',
  );
  $form['block1']['title'] = $form['title'];
  unset($form['title']);
  $form['block1']['body'] = $form['body'];
  unset($form['body']);
  $form['block1']['field_fichier'] = $form['field_fichier'];
  unset($form['field_fichier']);
  $output = drupal_render($form);
  return $output;
}

But the problem is the same.

******** EDIT 2 *********

I also noticed that the issue also happens when I try to use the default theme_fieldset() function as configuration:

$block1 = array(
        '#type' => 'fieldset',
        '#title' => t('Block 1'),
        '#weight' => -50,
        '#collapsible' => true,
        '#collapsed' => false,
        '#description' => t('Edit the welcome e-mail messages sent to new member accounts created by an administrator.'),
        '#tree' => TRUE,
        '#theme' => 'fieldset',
      );
      $form['block1'] = $block1;
      $form['block1']['body'] = $form['body'];

I double checked and the theme_fieldset() function (/includes/form.inc) is actually well called, but the fieldset is not generated as expected. Apparently something is missing so Drupal can do the job. May be the $variables object.

Here is a screenshot of what my form looks like with this configuration.

When I don't use the "#theme" config, everything is perfect:

$block1 = array(
    '#type' => 'fieldset',
    '#title' => t('Block 1'),
    '#weight' => -50,
    '#collapsible' => true,
    '#collapsed' => false,
    '#description' => t('Edit the welcome e-mail messages sent to new member accounts created by an administrator.'),
    '#tree' => TRUE,
  );
  $form['block1'] = $block1;
  $form['block1']['body'] = $form['body'];

Answer 1

You cannot use a theme function if you don't make Drupal know about it first. To do that, you need to implement hook_theme().

function mymodule_theme() {
  return array(
    'fieldset_custom3' => array(
      'render element' => 'element',
    ),
  );
}