WP Walker using custom css classes

Not sure you want to do it like this, but just in case, you can use the filter nav_menu_css_class :

function my_special_nav_class( $classes, $item ) {

   if ( is_single() && $item->title == 'Blog' ) {
      $classes[] = 'special-class';
   }

   return $classes;

}

add_filter( 'nav_menu_css_class', 'my_special_nav_class', 10, 2 );

You’ll find more example here