How do I theme a form element?
You should be able to wrap form elements with #prefix and #suffix:

$form['submit'] = array(
  '#type' =&gt; 'submit',
  '#value' =&gt; t('Submit'),
  '#attributes' =&gt; array('style' =&gt; "padding: 0 10px;"),
  '#theme' =&gt; "submit",
  '#prefix' =&gt; "&lt;div class='submit-button-custom'&gt;",
  '#suffix' =&gt; "&lt;/div&gt;",
);