Provide some really basic positioning styling

Problem/Motivation

Currently, we don't provide any styling to make the mega-menu content appear in a sensible location. Whilst we don't want to do anything that cannot be overridden by users of the module, it would be nice to at least be able to see the content under the owning menu item.

Proposed resolution

Provide the minimum viable styling to make it obvious how the module works.

Remaining tasks

Patch
Test

User interface changes

Styling change to position the mega-menu

API changes

none

Data model changes

none