The easiest way in Drupal to apply a consistent button class to form buttons is to use a custom HOOK_preprocess_button() function in the template.php that adds button and button variant classes (like the classic Bootstrap btn btn-VARIANT pattern).

As outlined here, this is a very easy pattern to implement. This works great for non-administration themes.

However, theme-level pre_preocess_ functions are called last. This implementation can wipeout transformations needed by Ctools to create administration interfaces.

I couldn't find a good example using hook_element_info_alter() to create this, so I thought I would post it here.

We will find the current button elements on the page, check to make sure they're not hidden, then use a #process callback function to apply Bootstrap button classes based on the text of the button.

Just drop all of these into your template.php (or a file included it in.)

Step one: Call hookelementinfo_alter()

  function theme_name_element_info_alter(&$type) {
    foreach ($type as &$element) {
      if (isset($element['#type'])) {
        if ($element['#type'] === 'button' || $element['#type'] === 'submit' ||  $element['#type'] === 'image_button') {
          $element['#process'][] = 'fixup_buttons_process';
        }
      }
   }
 }

Create 'fixup_buttons_process()' Callback

This is easy: take the current element and process it.

function fixup_buttons_process($element) {
  button_process($element);
  return $element;
}

sadfadsds

Create function to process button elements

This function checks the class array of the current element, if it doesn't contain our list of classes, it runs processes the current value of the button and adds the right class.

function button_process(&$item) {
  if (!is_array($item) || empty($item['#type'])) {
    return;
  }

  if (isset($item['#attributes']['class']) && btn_class_exists($item['#attributes']['class'])) {
    return;
  } else {
    $item['#attributes']['class'][] = 'btn';
    $item['#attributes']['class'][] = color_button($item['#value']);
  }
}

Check to see if there are existing classes, and then create them based on a library of words/class values

First we create a function checking the array of classes, which returns a boolean value.

function btn_class_exists($class_arr) {
  //load up our classes we want to check
  $classes = array(
      'btn-default',
      'btn-primary',
      'btn-success',
      'btn-info',
      'btn-warning',
      'btn-danger',
      //add as many as you'd like here!
  );

  if (empty($class_arr)) {
    return false;
  }

  $class_exists = array_intersect($classes, $class_arr);

  if (count($class_exists)) {
    return true;
  } else {
    return false;
  }

}

Now let's finally color the button!

Note: there are probably many more efficient ways of doing the checking for buttons, but I kept it simple here.

function color_button($string) {
   $values = array(
        'Save features'         => 'primary',
        'Mark as completed'     => 'primary',
        t('Create')             => 'success',
        t('Save')               => 'success',
        t('Write')              => 'success',
        'Unpublish'             => 'warning',
        t('Delete')             => 'danger',
        t('Remove')             => 'danger',
        //etc etc
    );

    $btn = ucfirst(strtolower($string));

    if (array_key_exists($btn, $texts)) {
      return 'btn-'.strtolower($texts[$btn]);
    } else {
      return 'btn-default';
    }
}