How to add CSS classes to buttons of Drupal Commerce checkout forms? How can I add a bootstrap btn-success class to the Drupal Commerce Checkout button in the cart form?

The Update cart button already has btn-info but I can't find btn-info anywhere in the code.

If would also like to adapt buttons in other parts as I expect btn-success buttons to lead to higher conversion.

Bootstrap has great classes, I don't want to fiddle with CSS to try and recreate that, I just want to use the Bootstrap classes. 

I'm okay with adding some PHP code to my theme's template.php, probably _form_alter() and this is probably the way to do it. But how exactly?
According that you have default views_form_commerce_cart_form_default form for your cart, this should do the job. Put the following lines in a custom module:

function MYMODULE_form_views_form_commerce_cart_form_default_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['actions']['checkout']['#attributes']['class'] = array('btn-success');
}


If not ok, check the ID of your form with 

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  drupal_set_message("Form ID is : " . $form_id); 
}


and apply the same above snippet with function MYMODULE_form_FORM_ID_alter.

EDIT : to apply btn-success to Continue buttons of the cart, checkout, review pages: 

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {

  switch ($form_id) {

    case 'views_form_commerce_cart_form_default':
      $form['actions']['checkout']['#attributes']['class'] = array('btn-success');
      break;

    case 'commerce_checkout_form_checkout':
    case 'commerce_checkout_form_review':
      $form['buttons']['continue']['#attributes']['class'] = array('btn-success');
      break;   
  }
}  


To do it yourself next time : in your hook form alter() module, check the form_id, dsm($form); and browse the dsm tree to find your needle...