I would like all my site pages to have a background-image, such as:

body {
  background-image: url(https://5e97-e8af726b4e97.wptiger.fr/wp-content/themes/twentyseventeen/assets/images/header.jpg);
  background-size: cover;
}

Using additional CSS field does not work.
What can be the solution, please?
Thank you