Theming a multistep form

Hi,

I'm trying to theme a multistep form. I have created a theming function for each step, but I'm having a lot of trouble getting the steps to pick up the appropriate theming function.

So far:

I have assigned each form step a theming function reference like so:

function first_step_of_multistep_form() {
        $form = array();
        (...)
	$form['#theme'] = 'theme_energy_leadgen_form';
	return $form;
}  

When I krumo out contents of form, the theming reference shows up in array

I have registered my theming functions with hook_theme:

function energy_leadgen_theme() {  
	return array(
		//Declaring theming function for all three steps in form
		'energy_leadgen_form' => array(
  		'render element' => 'form',
    ),
		'energy_leadgen_form_contact' => array(
  		'render element' => 'form',
    ),
		'energy_leadgen_form_payoff' => array(
	  	'render element' => 'form',
   ),
         (...)

I have created the different theming functions:
example - first theming function:

function theme_energy_leadgen_form($variables) {
	$form = $variables['form'];
	$output .= '<div class="intro_class">' . drupal_render($form['sprmaal']['intro'] . '</div>'); 
	$output .= drupal_render($form['sprmaal']['employees']); 
	$output .= drupal_render($form['sprmaal']['buildings']); 
	$output .= drupal_render($form['sprmaal']['energy']); 
	$output .= drupal_render($form['sprmaal']['measuring']); 
	$output .= drupal_render($form['sprmaal']['effort']); 	
	$output .= drupal_render($form['calculate']); 
	$output .= drupal_render_children($form);
	return $output;
}

But none of the HTML added within theming function seems to appear in rendered form. What am I doing wrong??? I have themed diff. elements within the form using this m.o. and this worked fine...