Problems registering a theme_hook

Hello, I am having problems registering a theme_hook for my module, which is my first, so i am still very new and learning the ropes. Here is the code I have, I am wondering if anyone could tell me why the theme function is not getting called?

function onin_menu() {
  $items = array();

  $items['volunteers'] = array(
    'title' => 'volunteers',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('onin_volunteers_page_form'),
    'access callback' => 'user_access',
    'access arguments' => array('access content'),
    'type' => MENU_CALLBACK,
  );

  return $items;
}

function onin_volunteers_page() {
  return drupal_get_form('onin_volunteers_page_form');
}

/**
 *Create the Volunteers Page Form
 */ 
function onin_volunteers_page_form(&$node, $form_state) {
$form['fullName'] = array(
    '#type' => 'textfield',
    '#title' => t('Enter your full name'),
    '#required' => TRUE,
  );
  $form['email'] = array(
    '#type' => 'textfield',
    '#title' => t('Enter your email'),
    '#required' => TRUE,
  );
  $form['phoneNumber'] = array(
    '#type' => 'textfield',
    '#title' => t('Enter your Phone Number'),
    '#required' => TRUE,
  );
  $form['interests'] = array(
    '#type' => 'textarea',
    '#title' => t('Enter your interests'),
  );
  $form['skills'] = array(
    '#type' => 'textarea',
    '#title' => t('Enter your skills'),
  );
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Save'),
  );
  return $form;
}

function onin_theme() {
  return array(
    'onin_volunteers_page_form' => array(
      'arguments' => array('form' => NULL ),
    )
  );
}


function theme_onin_volunteers_page_form($form) {
  $output = 'This should be added to the page but its not';
  $output .= drupal_render($form);
  return $output;
}

Thanks for your time. Also, I am open to other suggestions on how to improve my coding. Thanks