Form / Block / Template
If you want to theme a form, better override the form.html.twig template, because you don't know what happens in the background of this template and you don't want to do this all from scratch in your own template.

So put the complete form in the block:

public function build() {
  return \Drupal::formBuilder()-&gt;getForm('Drupal\boutique\Form\AjoutPanierForm');
}


And copy the form.html.twig in your template and change it.