Adding CSS files in child theme functions.php
add_action( 'wp_enqueue_scripts', 'myAddedCssFiles' );
function myAddedCssFiles() {
wp_enqueue_style(
'1AddedCss', get_template_directory_uri() . '/your-first-css-file.css', array( $parentStyle ), wp_get_theme()->get('Version') );
'2AddedCss', get_template_directory_uri() . '/your-second-css-file.css', array( $parentStyle ), wp_get_theme()->get('Version') );
/* copy and paste as many times as your css files
....
....
*/
'lastAddedCss', get_template_directory_uri() . '/last-css-file.css', array( $parentStyle ), wp_get_theme()->get('Version') );
}