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;
}