Nav accessibility

- Added aria tags to nav and dropdowns
- Minor style updates
  - Moved nav link color to a variable
  - Right side of section headers have a negative margin to better fit under heading
sorry just want to make sure aria roles are being added with care and proper functionality. If you're adding has-popup then there might need to be a few extra aria attributes added for when the menu is expanded for example, as well as ensuring the menu can be accessed via keyboard alone.