People searching through this forum wondering why this mod its not working; stylesheet_categories_menu.css code gets messed up when you upload it; open this file on your server and you will see
/includes/templates/classic/css/stylesheet_categories_menu.css
or
/includes/templates/YOUR_CUSTOM_TEMPLATE_NAME/css/stylesheet_categories_menu.css

you need to open stylesheet_categories_menu.css on your computer copy the CSS it and go your to FTP or filemanager and edit the file on your server and past it into the file, deleting the scrambled code in it.
Or recreate the file from scratch by copying the CSS in to notepad or something and saving the note pad file as stylesheet_categories_menu.css and upload it again.

Someone could add an update with the correct coding for stylesheet_categories_menu.css