custom.css wordt niet herkent na registreren en enqueue volgens de instructies

hallo,
ik heb custom.css bestand gemaakt naast mij style.css en geuploaded naar de theme directory.
ik probeer nu deze css bestand regitreren en enqueue volgens:
https://codex.wordpress.org/Function_Reference/wp_register_style example code:

/ Register style sheet.
add_action( 'wp_enqueue_scripts', 'register_plugin_styles' );

/**
 * Register style sheet.
 */
function register_plugin_styles() {
	wp_register_style( 'my-plugin', plugins_url( 'my-plugin/css/plugin.css' ) );
	wp_enqueue_style( 'my-plugin' );
}

nu mij code is voor custom.css

*
 * Register style sheet.
 */
function register_custom_styles() {
	wp_register_style( 'developer', get_template_directory_uri() .  '/http://webdesignleren.net/wp-content/themes/developer/custom.css' );
	wp_enqueue_style( 'developer' );
}
 add_action( 'wp_enqueue_scripts', 'register_custom_styles' );

maar de custom.css wordt niet herkent in WP.
maar als ik gebruik maak van link rel property in header.php het wordt wel herkent door WP.
ik heb in Google voor oplossing gezocht zonder resultaat.
mij functions.php code is als volgende:

if ( ! function_exists( 'developer_setup' ) ) :
/**
 * Sets up theme defaults and registers support for various WordPress features.
 *
 * Note that this function is hooked into the after_setup_theme hook, which
 * runs before the init hook. The init hook is too late for some features, such
 * as indicating support for post thumbnails.
 */
function developer_setup() {
	/*
	 * Make theme available for translation.
	 * Translations can be filed in the /languages/ directory.
	 * If you're building a theme based on sinan, use a find and replace
	 * to change 'sinan' to the name of your theme in all the template files.
	 */
	load_theme_textdomain( 'developer', get_template_directory() . '/languages' );

	// Add default posts and comments RSS feed links to head.
	add_theme_support( 'automatic-feed-links' );

	/*
	 * Let WordPress manage the document title.
	 * By adding theme support, we declare that this theme does not use a
	 * hard-coded <title> tag in the document head, and expect WordPress to
	 * provide it for us.
	 */
	add_theme_support( 'title-tag' );

	/*
	 * Enable support for Post Thumbnails on posts and pages.
	 *
	 * @link https://developer.wordpress.org/themes/functionality/featured-images-post-thumbnails/
	 */
	add_theme_support( 'post-thumbnails' );

	// This theme uses wp_nav_menu() in one location.
	register_nav_menus( array(
		'primary' => esc_html__( 'Primary', 'developer' ),
	) );

	/*
	 * Switch default core markup for search form, comment form, and comments
	 * to output valid HTML5.
	 */
	add_theme_support( 'html5', array(
		'search-form',
		'comment-form',
		'comment-list',
		'gallery',
		'caption',
	) );

	// Set up the WordPress core custom background feature.
	add_theme_support( 'custom-background', apply_filters( 'developer_custom_background_args', array(
		'default-color' => 'ffffff',
		'default-image' => '',
	) ) );
}
endif;
add_action( 'after_setup_theme', 'developer_setup' );

/**
 * Set the content width in pixels, based on the theme's design and stylesheet.
 *
 * Priority 0 to make it available to lower priority callbacks.
 *
 * @global int $content_width
 */
function developer_content_width() {
	$GLOBALS['content_width'] = apply_filters( 'developer_content_width', 640 );
}
add_action( 'after_setup_theme', 'developer_content_width', 0 );

/**
 * Register widget area.
 *
 * @link https://developer.wordpress.org/themes/functionality/sidebars/#registering-a-sidebar
 */
function developer_widgets_init() {
	register_sidebar( array(
		'name'          => esc_html__( 'Sidebar', 'developer' ),
		'id'            => 'sidebar-1',
		'description'   => esc_html__( 'Add widgets here.', 'developer' ),
		'before_widget' => '<section id="%1$s" class="widget %2$s">',
		'after_widget'  => '</section>',
		'before_title'  => '<h2 class="widget-title">',
		'after_title'   => '</h2>',
	) );
}
add_action( 'widgets_init', 'developer_widgets_init' );

/**
 * Add SVG capabilities
 */
function developer_theme_svg_mime_type( $mimes = array() ) {
  $mimes['svg']  = 'image/svg+xml';
  $mimes['svgz'] = 'image/svg+xml';
  return $mimes;
}
add_filter( 'upload_mimes', 'developer_theme_svg_mime_type' );
//Register style sheet.

/**
 * Register style sheet.
 */
function register_custom_styles() {
	wp_register_style( 'developer', get_template_directory_uri() .  '/http://webdesignleren.net/wp-content/themes/developer/custom.css' );
	wp_enqueue_style( 'developer' );
}
 add_action( 'wp_enqueue_scripts', 'register_custom_styles' );

/**
 * Enqueue scripts and styles.

   function developer_scripts() {  
wp_enqueue_style( 'developer-style', get_stylesheet_uri() );
    
	wp_enqueue_script( 'developer-navigation', get_template_directory_uri() . '/js/navigation.js', array(), '20151215', true );

	wp_enqueue_script( 'developer-skip-link-focus-fix', get_template_directory_uri() . '/js/skip-link-focus-fix.js', array(), '20151215', true );

	if ( is_singular() && comments_open() && get_option( 'thread_comments' ) ) {
		wp_enqueue_script( 'comment-reply' );
	}
}
add_action( 'wp_enqueue_scripts', 'developer_scripts' );

/**
 * Implement the Custom Header feature.
 */
require get_template_directory() . '/inc/custom-header.php';

wat kan het probleem zijn of wat ik doe fout hier ,ik wil weten waarom deze code wordt niet herkent door WP ?
Dank U Wel
johannes