how add class in containers

I don’t understand using rigid widgets having the ability to edit the theme. So, I call attention to the wp_nav_menu() for flexibility improvement.

functions.php:

<?php

// register menu in `side_menu`
register_nav_menu( 'side_menu', __( 'Sidebar', 'theme-slug' ) );

Now you can build your own menu under Appearance -> Menus.

Use wp_nav_menu() function to display your menu in the header.php:

<?php

$args = array(
    'theme_location'  => 'side_menu',
    'menu'            => 'side_menu',
    // here goes the container
    'container'       => 'div',
    // here goes the container classes
    'container_class' => 'menu-side-menu-container menu-sidebar',
    // more arguments if different fron default
    // See the link at the end
);

wp_nav_menu($args);

For more information see:

• register_nav_menu()
• wp_nav_menu()
• nav_menu_css_class filter