Prestashop is a good and mostly free open source online shop solution. Here’s the info where to find latest top menu module to use with it.
On back-end Modules install Menu v2.8.6 by Julien Breux module found in Front Office Modules.
Then go to Configure link and configure. Here you may create a custom top link for dropdown categories of products (functionality badly needed for PrestaShop, by default it will be not translatable Home).
Style with CSS. You may create jbx_menu folder in your theme CSS folder, in Modules (/themes/yourtheme/css/modules/jbx_menu/css/superfish-modified.css will be the right location).