In this article, I'll show you how to add Twig Template Suggestions for fieldset Elements in drupal 8 with hook_theme_suggestions_fieldset_alter():
/**
* @param array $suggestions
* @param array $variables
* @param $hook
*/
function THEMENAME_theme_suggestions_fieldset_alter(array &$suggestions, array $variables, $hook)
{
if (isset($variables['element']['#id'])) {
$id = str_replace("-", "_", $variables['element']['#id']);
$suggestions[] = $hook . '__' . $id;
}
}
