Creating a Mega Menu with the Advanced Menu Module
HubSpot doesn't have any out of the box functionality with mega menus. However, there are a couple of options that may work for you.
1. The Advanced Menu editor (Content > Content Settings > Advanced Menus) supports adding HTML markup to the label fields. So you could wrap your text in divs or spans with custom classes, which you allow you to apply custom styling. You can also insert img tags right into that field. In most cases, you can get the desired look and feel for your mega menu using this option and it allows you to use only the Advanced Meun module
2. For further customization or often when copying the menu over from another domain, a workaround is to use Rich Text/Custom HTML modules to create the submenus for your main advanced menu and then use CSS to style those.