Change existing configuration at theme installation
i've found a solution: define the style of the image directly into the twig  template. As a theme can define new style of image when it is installed it's possible to use a non native style of image. In the example below i use the style "wide", created at the theme installation, instead of the native style "medium" used in the indexed content in the homepage.

 {% for item in content.field_image['#items'] %}
  {% set image = {
    '#theme':      'image_style',
    '#style_name': 'wide',
    '#uri':        item.entity.uri.value,
    '#alt':        item.alt,
    '#width':      item.width,
    '#height':     item.height
  } %}
  {{ image }}