Revert main nav font size change

[froboy]

Related Issue/Ticket:

Community request

Steps to test:

• [ ] Observe VY main nav font size matches the rest of the site

Quality checks:

Please check these boxes to confirm this PR covers the following cases:

• Maintaining our upgrade path is essential. Check one or the other:
  • [ ] This PR provides updates via hook_update_N or other means.
  • [ ] No updates are necessary for this change.
• Front end fixes should be tested against all of the Open Y Themes.
  • [ ] Tested against Carnation
  • [ ] Tested against Lily
  • [ ] Tested against Rose
  • [ ] This change does not contain front-end fixes.
• [ ] I have flagged this PR "Needs Review" or pinged the VY devs/QA team in Slack