theming username field in user-register form
As you can see on the Form API reference page, '#theme' Only affects the input box itself. To remove other things generated by the form API, you'll have to clear the array -

$form['account']['name']['#title'] =&gt; '';
$form['account']['name']['#description'] =&gt; '';


or just

$form['account']['name'] = array(...);


To reset existing elements.