drupal 7 form theme function not being called - drupal-7

Question

I'm trying to register a theme function for a simple form in a custom module, but the theme function is not being called. I just get the basic form.
Here's my hook_theme():
function kss_membership_theme($existing, $type, $theme, $path){
$items = array();
$items['kss_membership_payment_form'] = array(
'render element' => 'form',
);
return $items;
}
for this form:
/**
* Returns the form for the second page of the payment process
*/
function kss_membership_payment_form($form, &$form_state) {
$form['description'] = array(
'#type' => 'item',
'#title' => t('We currently accept Paypal payments'),
);
$form['submit'] = array(
'#type' => 'submit',
'#value' => t('Submit'),
'#submit' => array('kss_membership_payment_form_submit'),
);
$form['#theme'] = array('theme_kss_membership_payment_form');
return $form;
}
and here is the theme function:
function theme_kss_membership_payment_form($variables) {
// Isolate the form definition form the $variables array
$form = $variables['form'];
$output = '<h2>' . t('Please enter your information below') . '</h2>';
$output .= '<div id="personal_details">';
$output .= drupal_render($form['description']);
$output .= drupal_render($form['submit']);
// Additional template output goes here....
$output .= '</div>';
$output .= drupal_render_children($form);
return $output;
}

Answer 1

You are very near to solution, Only one problem is there.
theme calling is wrong
$form['#theme'] = array('theme_kss_membership_payment_form');
you are required to call
$form['#theme'] = array('kss_membership_payment_form');
After that You must required to clear the cache from the admin => configuration => Performance => Clear Cache button.
/**
* Returns the form for the second page of the payment process
*/
function kss_membership_payment_form($form, &$form_state) {
$form['description'] = array(
'#type' => 'item',
'#title' => t('We currently accept Paypal payments'),
);
$form['submit'] = array(
'#type' => 'submit',
'#value' => t('Submit'),
'#submit' => array('kss_membership_payment_form_submit'),
);
$form['#theme'] = array('kss_membership_payment_form');
return $form;
}