I was thinking this might be done by adding custom CSS to a page template. I have a page template like this:

<?php
/*
 * Template Name: Grafy Full Width
 */

/*add_filter('materialis_full_width_page', '__return_true');*/

materialis_get_header();
?>
    <div <?php echo materialis_page_content_atts(); ?>>
        <div class="grafy-full-width-content">
            <?php
            while (have_posts()) : the_post();
                the_content();
            endwhile;
            ?>
        </div>
    </div>

<?php get_footer(); ?>

And I thought I might be able to add the CSS like this (basically I have 10 main tags and all I need to do is to change the header background, so I might just add 10 one-lined CSS files and condition them like this):

<?php function wpse_enqueue_page_template_styles() {
    if (has_tag('Lesy')) {
        wp_enqueue_style( 'Lesy', get_template_directory_uri() . '/css/Lesy.css' );
    }
}
add_action( 'wp_enqueue_scripts', 'wpse_enqueue_page_template_styles' );?>

No matter what I do, this just isn’t working. What could I be doing wrong?
Many thanks!