To change any wp core file

To change any wp core file

// Custom login
function custom_login() {
        $files = '<link rel="stylesheet" href="'.get_bloginfo('template_directory').'/css/login.css" />';
        echo $files;
}
add_action('login_head', 'custom_login');

Or=======


function my_login_stylesheet() {
    wp_enqueue_style( 'custom-login', get_template_directory_uri() . '/style-login.css' );
}
add_action( 'login_enqueue_scripts', 'my_login_stylesheet' );