Sometimes, you want to style your menus further but the current features of your mega menu builder doesn't give you ability to do this. That's when you need to add custom CSS.
In this tutorial, we'll show you how to add custom CSS to your menu in Ninja Menus, an easy and powerful Magento 2 mega menu extension.
https://www.magezon.com/magento-2-mega-menu.html

Supposed that you have already created a menu using Ninja Menus. Open the edit page of the menu. Then open General > Advanced Settings. Here you'll find CSS Classes and Custom CSS fields where you can add your custom CSS.

Remember to save after finishing.