Add a class to a paragraph

https://www.scottsawyerconsulting.com/blog/Component-Driven-Drupal-Them…

hasField('field_button_color') && !$paragraph->field_button_color->isEmpty()) {
    $variables['attributes']['class'][] = 'button-color--' . $paragraph->field_button_color->value;
  }
}

or in

**
 * Prepares variables for entity template.
 *
 * Default template: items.html.twig.
 *
 * @param array $variables
 *   An associative array of variables.
 */
function template_preprocess_entity(&$variables) {
  $field_attribute_mapping = [
        'field_placement' => 'overlay-placement',
    ];

    foreach ($field_attribute_mapping as $field_name => $attribute) {

        if (!$entity->hasField($field_name)) {
            continue;
        }

        if ($entity->get($field_name)->isEmpty()) {
            continue;
        }

        if ($value = $entity->get($field_name)->target_id) {
            // This is an entity reference field. Get the entity label.

            // Get the related entity (taxonomy term).
            if ($related_entity = $entity->get($field_name)->entity) {
                $entity_label = $related_entity->label();
                $value = \Drupal\Component\Utility\Html::cleanCssIdentifier($entity_label);
            }
            else {
                // Fall back to entity ID retrieved above.
            }

        }
        else {
            $value = $entity->get($field_name)->value;
        }

        $variables['attributes']['class'] = "{$attribute}--{$value}";
        $variables['attributes']["data-{$attribute}"] = $value;
  }



}