Drupal - Use custom content entity theme suggestions in own module?

To make a theme suggestion work in the module folder you need an additional theme hook:

function offer_theme($existing, $type, $theme, $path) {
  return [
    'offer' => [
      'render element' => 'elements',
    ],
    'offer__full' => [
      'base hook' => 'offer',
    ],
  ];
}