For the button you should use $form['#submit']
.
In any case you can use hook_form_alter() and devel's dpm($form)
to get the form data. A generic example:
function MYTHEME_form_alter(&$form, &$form_state, $form_id) {
dpm($form);
}
The submit button may be accessed like this for a common Drupal theme (structure taken from the dpm() function):
$form['actions']['submit']['#attributes']['tabindex'][] = "2";
One important thing here is to be careful because you may override the form again later on your theme... In this case look for the very last hook and apply changes there.