Hi Kees,
You can do so by enqueueing your own style in the Block Editor with the Child theme enabled.
For instance, in your Child Theme directory, you create a new file called my-editor-style.css
Then, add this PHP in functions.php:
add_action( 'enqueue_block_editor_assets', function() {
wp_enqueue_style( 'my-block-editor-styles', get_stylesheet_directory_uri() . "/my-editor-style.css", false,'1.0', 'all' );
} );
Now, in your my-editor-style.css file, you can add CSS like this:
p {
color: #000 !important;
}