Sorry for this delay, it’s been our Easter the past few days and I just wasn’t able to reply earlier!
Admin tell to add this :
jQuery("#menu-main-menu").addClass('sm');
This should be added to some of the JS files that your theme uses. To be more precise, it should be like this so it is executed ondomready:
jQuery(function() {
jQuery("#menu-main-menu").addClass('sm');
});
and this :
<?php wp_nav_menu( array( 'menu' => '...', 'menu_id' => 'menu-main-menu', 'menu_class' => 'sm' ... ) ); ?>
This should be added in the PHP include file that currently outputs your main menu. It might possibly be “header.php” but I am not familiar with the theme you use so it might be some other file.