$build ['send_data'] = array(
  '#theme' => 'my_custom_template',
  '#content' => $result,
);

function my_custom_theme($existing, $type, $theme, $path) {
  return [
    'my_custom_template' => [
      'variables' => ['content' => NULL,]
    ],
  ];
}

function my_custom_theme($existing, $type, $theme, $path) {
  return [
    'my_custom_template' => [
      'variables' => ['content' => NULL]
    ],
    'user' => [
      'variables' => ['content' => NULL]
    ],
  ];
}