class.php

class f6_dropdown_menu extends Walker_Nav_Menu {
  // Add vertical menu class and submenu data attribute to sub menus
  function start_lvl( &$output, $depth = 0, $args = array() ) {
    $indent = str_repeat( "\t", $depth );
    $output .= "\n$indent<ul class=\"vertical menu dropdown\">\n";
  }

function.php

function primaryNavigation() {
    if (has_nav_menu('primary_navigation')) {
      return  wp_nav_menu([
        'container' => false,
        'menu' => __( 'Primary Navigation', 'sage' ),
        'menu_class' => 'horizontal menu dropdown align-right',
        'theme_location' => 'primary_navigation',
        'items_wrap'      => '<ul id="%1$s" class="%2$s" data-dropdown-menu>%3$s</ul>',
        // Recommend setting this to false, but if you need a fallback....
        'fallback_cb'    => false,
        'walker'         => new \f6_dropdown_menu()
      ]);
    }
    return '<span class="">Please select a Primary Menu</span>';
  }

The first file, the walker class, adds css classes to the submenus.

The second file adds classes to the menu.

I'd like to rewrite the function as function primaryNavigation( $menuclass $submenuclass ) and pass the params to the function and walker class respectively.