BobVolte

I was able to find a working solution. I explain it, in case it might be useful for someone. This is the functions.php file of my child theme :

<?php

add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );
function theme_enqueue_styles() {
    wp_enqueue_style( 'style', get_template_directory_uri() . '/style.css' );
	wp_enqueue_style('tortuga-flexslider', get_template_directory_uri(). '/css/flexslider.css');
	wp_enqueue_style('flexslider', get_stylesheet_directory_uri(). '/css/flexslider.css');
}

I enqueue my parent theme flexslider.css file, and then in enqueue the flexslider.css of the child theme, using get_stylesheet_directory_uri().

I still have a question though : I’m enqueueing the parent flexslider.css file, but it was already enqueued in the inc/slider.php file. Is this redundant ? Is there another way to do this, or is my solution acceptable ?

Nevermind, I found it already ??

Thanks anyway…!