Adding Custom css file

Put this code in functions.php file for add new css file (more info here).

add_action( 'wp_enqueue_scripts', 'enqueue_my_styles' );
function enqueue_my_styles() {
    wp_enqueue_style( 'my-theme-ie', get_stylesheet_directory_uri() . "/css/ie.css");
}
  1. if using a parent theme then put code in parent theme function.php file and use get_template_directory_uri()
  2. if using a child theme then put code in child theme function.php file and use get_stylesheet_directory_uri()

Note: Relative CSS path must be correct.