Glossary needs styling

[jessecodescode]

This issue came out of this issue.

The glossary button, toggled glossary, and glossary content can all get pretty.

[elinaru]

Proposed mock-ups for are here: https://invis.io/M3PL9PAY8XP

[theecrit]

Just popping my head in here to suggest considering different language for the navigation label. I wonder if "Definitions" might be a little more plain language (a la https://www.plainlanguage.gov/about/definitions/ ) for non-native English speakers.