cocconのようなカスタムCSSやカスタムJavaScriptを追加する【自作】

//Custom CSS Widget
add_action( 'admin_menu', 'custom_css_hooks' );
add_action( 'save_post', 'save_custom_css' );
add_action( 'wp_head','insert_custom_css' );
function custom_css_hooks() {
  add_meta_box( 'custom_css', 'Custom CSS', 'custom_css_input', 'post', 'normal', 'high' );
  add_meta_box( 'custom_css', 'Custom CSS', 'custom_css_input', 'page', 'normal', 'high' );
}
function custom_css_input() {
  $post = get_queried_object();
  echo '<input type="hidden" name="custom_css_noncename" id="custom_css_noncename" value="'.wp_create_nonce('custom-css').'" />';
  echo '<textarea name="custom_css" id="custom_css" rows="5" cols="30" style="width:100%;" placeholder="このページにだけ適用するCSSコードを入力します。styleタグは不要です。">'.get_post_meta($post->ID,'_custom_css',true).'</textarea>';
}
function save_custom_css($post_id) {
  if ( !wp_verify_nonce( $_POST['custom_css_noncename'], 'custom-css' ) ) return $post_id;
  if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE) return $post_id;
  $custom_css = $_POST['custom_css'];
  update_post_meta( $post_id, '_custom_css', $custom_css );
}
function insert_custom_css() {
  if ( is_page() || is_single() ) {
    if ( have_posts() ) : while ( have_posts() ) : the_post();
      echo '<style>' . get_post_meta(get_the_ID(), '_custom_css', true) . '</style>';
    endwhile; endif;
    rewind_posts();
  }
}

//Custom JS Widget
add_action( 'admin_menu', 'custom_js_hooks' );
add_action( 'save_post', 'save_custom_js' );
add_action( 'wp_footer','insert_custom_js' );
function custom_js_hooks() {
  add_meta_box( 'custom_js', 'Custom JavaScript', 'custom_js_input', 'post', 'normal', 'high' );
  add_meta_box( 'custom_js', 'Custom JavaScript', 'custom_js_input', 'page', 'normal', 'high' );
}
function custom_js_input() {
  $post = get_queried_object();
  echo '<input type="hidden" name="custom_js_noncename" id="custom_js_noncename" value="'.wp_create_nonce('custom-js').'" />';
  echo '<textarea name="custom_js" id="custom_js" rows="5" cols="30" style="width:100%;" placeholder="このページにだけ適用するJavaScriptコードを入力します。scriptタグは不要です。">'.get_post_meta($post->ID,'_custom_js',true).'</textarea>';
}
function save_custom_js($post_id) {
  if (!wp_verify_nonce($_POST['custom_js_noncename'], 'custom-js')) return $post_id;
  if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) return $post_id;
  $custom_js = $_POST['custom_js'];
  update_post_meta($post_id, '_custom_js', $custom_js);
}
function insert_custom_js() {
  if ( is_page() || is_single() ) {
    if ( have_posts() ) : while ( have_posts() ) : the_post();
      echo '<script>' . get_post_meta(get_the_ID(), '_custom_js', true) . '</script>';
    endwhile; endif;
    rewind_posts();
  }
}