custom_user.module

<?php 

if(!defined('LOGO_MAIL')){
  define('LOGO_MAIL', base_path().drupal_get_path('module', 'custom_user')."/images/logo-mail.png");
}

/**
 * Implements hook_theme().
 **/
function custom_user_theme($existing, $type, $theme, $path) {
  return [
    'user_prehome' => [
      'variables' => ['intro' => '', 'loginForm' => [], 'registerForm' => []],
    ],
    'page_prehome' => [
      'variables' => ['intro' => '', 'content' => []],
    ],
    'user_mail_tpl' => [
      'variables' => ['logo' => '', 'title' => '', 'greeting' => '', 'introduction' => '', 'contenu' => [], 'conclusion' => ''],
    ],
  ];
}

/**
 * Implements hook_form_alter().
 **/
function custom_user_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
    global $user;
    switch ($form_id) {
        // login form 
        case 'user_login_form':
        break;
        // register form
        case 'user_register_form':
          $admin_context = \Drupal::service('router.admin_context');
          if (!$admin_context->isAdminRoute()) {
            unset($form['account']['status']);
            unset($form['account']['roles']);
            unset($form['field_date_de_naissance']);
            unset($form['field_mobile']);
            unset($form['field_nom']);
            unset($form['field_prenom']);
          }
        break;
        default:
        break;
    }
}

/**
* Implements hook_mail().
*/
function custom_user_mail($key, &$message, $params) {
 $options = array(
   'langcode' => $message['langcode'],
 );
 switch ($key) {
   case 'accuse_inscription_fidele':
     $message['from'] = \Drupal::config('system.site')->get('mail');
     $message['subject'] = t('Accus&eacute; de reception de votre demande d\'inscription');
     $message['body'][] = $params['message'];
     break;
 }
}

function myprint($var=NULL,$die=TRUE){
  print '<pre>';
  print "---------START--------";
  var_dump($var);
  if($die){ die("die"); }else{ print '---------END---------'; }
}