place a direct link to custom-stylesheet, under appearance menu in wp-admin

You can check out this excellent answer by @Eugene Manuilov. In your case the relevant admin page action is:

load-appearance_page_customstyle

and the url to the custom stylesheet you want to edit:

get_admin_url().'theme-editor.php?file=custom-stylesheet.css&theme=". get_stylesheet()."&scrollto=0';

Then your code example would be:

add_action('admin_menu', 'add_appearance_menu');    
function add_appearance_menu() {
    add_submenu_page( 'themes.php', 'Custom Stylesheet', 'customstyle', 'manage_options', 'customstyle', '__return_null'); 
}

add_action( 'load-appearance_page_customstyle', 'custom_redirect' );
function custom_redirect() {
    if ( 'customstyle' === filter_input( INPUT_GET, 'page' ) ) {
        $file2edit = "custom-stylesheet.css"; // change this to your needs
        $location = get_admin_url().'theme-editor.php?file=".$file2edit."&theme=". get_stylesheet()."&scrollto=0';
        wp_redirect( $location, 301);
        exit();
    }
}

When you click the submenu link

http://example.com/wp-admin/themes.php?page=customstyle

the hook load-appearance_page_customstyle from

do_action('load-' . $page_hook);

in /wp-admin/admin.php is activated with the redirect defined above.