Add a post-type specific editor stylesheet to each post edit screen in WordPress back-end. See https://sebastien-meric.com/wordpress-gutenberg-feuille-de-style-specifique-type-de-contenu-post-type/ (in french).

custom-post-type-specific-editor-style.php

<?php
/**
 * Registers a specific editor stylesheet for the current post type.
 *
 * @see https://developer.wordpress.org/reference/functions/add_editor_style/
 */
function my_theme_add_editor_styles() {
    global $pagenow, $current_screen;

    // Current post-type
    $post_type = $current_screen->post_type ? $current_screen->post_type : '';
    // Custom post types
    $custom_post_types = get_post_types( array( 'public' => true, '_builtin' => false ) );
    // Default post and page post types
    $custom_post_types['post'] = 'post';
    $custom_post_types['page'] = 'page';

    if ( in_array( $post_type, $custom_post_types )
            && ( 'post-new.php' === $pagenow || 'post.php' === $pagenow )
            && file_exists( get_stylesheet_directory() . '/editor-style-' . $post_type . '.css' )
    ) {
        add_editor_style( get_stylesheet_directory_uri() . '/editor-style-' . $post_type . '.css' );
    }
}
add_action( 'current_screen', 'my_theme_add_editor_styles' );
?>