There are several utility classes, available through the template.
When creating a module instance of type 'Menu', go to the Advanced tab and enter the classes in the parameter 'Menu class'.
overbg Will switch the menu items' text color from black to white so text can remain visible.
pre For raw menus, will add expanding sub-menus functionality, with icon clickable in front of the menu item that can expand.
post For raw menus, will add expanding sub-menus functionality, with icon clickable behind the menu item that can expand.
2.0 Bootstrap classes can be used as well.
For instance, use Bootstrap 5's
flex-column to force menu items into a single column (menus are built and displayed via flexbox).