There’s no way of doing this out-of-the-box at the moment, but it’s possible. I’d recommend a few things:
1) If you don’t already: learn more about HTML, CSS flexbox, CSS grids, and browser Inspector tools.
I’d recommend checking out FreeCodeCamp, Mimo, and SuperHi’s Plan Design Code courses. Give yourself a month, pratice every day, and you’ll get the hang of things eventually. You can even learn by inspecting the webpage you sent. From what I can see, their menu makes use of CSS grid and special class names.
2) Once you get the hang of things, play around with Publii’s submenu functionality
(See image) Adding menus and submenus will allow you to access new class names. Each publii theme has a different way of displaying submenus.
3) Finally: Use CSS classes to create the columns and change the appearance.
This is where knowledge of HTML and CSS comes in handy. Publii allows you to add custom class names for each menu item. Using these class names, you can push customizations further via Custom CSS or by modifying main.css.
This is what I did to have two different looks for the main menu items of my site diasporamemory.com