btn class needed in order to override primary/secondary button colors

Yeah, the current implementation is a bit ugly and done quickly.

> $variables['element']['wizard_prev']['#attributes']['class'][] = 'custom-button-ghost';

I am not sure what is custom-button-ghost here i guess it is a custom class for a specific project?

> What is also strange, is that we have two different attributes/classes, btn-primary and button--primary and that just adds extra confusion.

Yes because Drupal core is saying that it's primary button is button--primary and Bootstrap is saying btn-primary. So we have two conventions here.
Current input--submit.html.twig template is trying to solve these two conventions and simplify usage for 'most' of use cases. Currently, it supports core button--primary, button--danger CSS classes and treats all other cases as Bootstrap secondary button. Meaning this should work for regular Drupal forms - proper Bootstrap CSS classes will be attached, etc.

I am not sure how to solve this conflict properly? You want to support a ghost button but that is not something that Drupal is defining, or at least it didn't do it before, right? Maybe something changed with Claro admin theme?

The easiest way is to attach 'btn' class to custom cases and then attach your additional custom CSS classes. This worked for now for other custom cases. You anyway need to define preprocessor and inject your custom CSS class, so adding a btn class is not a big deal.

Now if you want to just attach custom-button-ghost CSS class and not btn i guess you would not be able to do right now with the current template, you would need to override it in child theme. IMHO overriding template is fine in this case because this is a Bootstrap base theme and by default, it should always attach btn base class.

Hopes these informations are clearing stuff a bit. I am open to suggestions on how to improve this.

> Otherwise, we need to document this somewhere for future reference.

If this is a Bootstrap based theme then it would make sense that by default all buttons use Bootstrap btn base class for that.

> That said, maybe adding a key class/attribute like button--custom when defining "custom-button-ghost" in the preprocess, for example, and checking for that in the template would somehow make sense?

If we really want to do this I don't think we should use class/attribute for this but inject custom variable in preprocessor and then check in the template should we use default logic or just pass through. How does this sound to you?

Webform submodule webform_bootstrap is doing something similar in `_webform_bootstrap_convert_button_classes()`:

function _webform_bootstrap_convert_button_classes(array &$classes) {
  $drupal_to_bootstrap = [
    // Convert Drupal's button classes to Bootstrap's btn classes.
    'button-action' => 'btn-primary',
    'button--small' => 'btn-sm',
    'button--primary' => 'btn-primary',
    'button--danger' => 'btn-danger',
    'button' => 'btn',
  ];
  foreach ($classes as $index => $class) {
    if (isset($drupal_to_bootstrap[$class])) {
      $classes[$index] = $drupal_to_bootstrap[$class];
    }
  }
}

And this is called in hook_link_alter() and template_preprocess_file_managed_file(). Similar but a bit different approach and done in PHP instead of template.

Not sure about this. Changing anything in this logic could break now custom themes, and we do not want to do it for 1.x.

The `btn` CSS class is a base CSS class for a Bootstrap button so it kinda make sense that if you want to add some custom button class you should also add `btn`.

On the other hand having button button--primary and btn btn-primary on the same element is a bit too much.

Let's move this to 2.x version with a two possible options to solve this:
- Instead of converting to Bootstrap buttons maybe we define Drupal button classes to work like Bootstrap classes - this will be easy with applying mixins or using SASS @extend rule. This would allow us to leave core input--submit template. Not sure will Bootstrap like this, we need to test.
- If first option does not work let's do then like webform is doing by converting Drupal button classes to appropriate Bootstrap classes.