Theming forms on alter with Drupal 7 - php

Question

I'm having issues customizing a radio button for a specific form using hook_theme. Below is the piece of code I have on my module; see my comments elaborating the issue I encounter:
// Implementation of hook_form_alter().
function mymodule_form_alter(&$form, $form_state, $form_id){
// e.g form id: commerce_cart_add_to_cart_form_u6onPJSgS7pOgw0Tlo7zHy42LTQzbV913taANkYQKTo
if (strpos($form_id, 'commerce_cart_add_to_cart_form') !== FALSE) {
// Alter add to cart form
mymodule_commerce_cart_add_to_cart_form_alter($form, $form_state, $form_id);
}
}
function mymodule_commerce_cart_add_to_cart_form_alter(&$form, $form_state, $form_id) {
// Change the field type to radios.
$form['attributes']['field_artwork_ref']['#type'] = 'radios';
// Apply my custom theme for radios.
$form['attributes']['field_artwork_ref']['#theme'] = array('custom_radios');
}
// Implementation of hook_theme().
function mymodule_theme() {
return array(
'custom_radios' => array(
'variables' => array('element' => NULL),
),
);
}
function theme_custom_radios($variables) {
// Custom theme should go here.
// However, $variables are empty, print_r gives me "Array ( [element] => )."
// I am at least expecting to see my radio element here.
print_r($variables);
}

Answer 1

Themes for Drupal 7 form elements need to use the new render array key instead of variables in the theme definition:
function mymodule_theme() {
return array(
'custom_radios' => array(
'render element' => 'element',
),
);
}
Once you've made the change clear Drupal's caches and your code should work (I've just tested the above and it works fine).