How to apply a theme function/template to my form

Question

I think theme_form() is responsible for rendering forms! I want to change the look of a form, a lot. I think just instead of theme_form I can use my custom theme function/template file.

I can use hook_form_alter to change any form's structure using my module. Now my question is, how do tell the form to use a theme function/template I've created?

Is there a #theme property (or something like that) available for forms, which I can set to my theme function/template file?

P.S. I checked out this but I need more explanation, when the case is: I'm trying to theme a form defined by another module/core modules.

Answer 1

Open template.php of your theme

function YOURTHEMENAME_theme($existing, $type, $theme, $path) {
  $hooks['form_id_of_form'] = array(
    'template' => 'path_to_tpl_file',
    'arguments' => array('form' => NULL),
  );
  return $hooks;
}

Also you can try

function hook_form_alter(&$form, &$form_state, $form_id) {
  if($form_id == 'form_id_of_form') {
    $form['#theme'] = 'path_to_tpl_file';
  }
}

Answer 2

Is there a #theme property (or something like that) available for forms, which I can set to my theme function/template file?

Yes there is. Take a look at the #theme key in the Form API Reference. Take a look at the code for theme_form() for the default implementation.