Add mobile styles for core/navigation block

Changes proposed in this pull request

This PR adds styling that is applied to the main nav mobile menu.

Pre-submit checklist

As the author of this pull request, I verify that:

[ ] I have set the target branch to master.
[ ] I have detailed the purpose of this Pull Request in a non-technical way.
[ ] I have detailed how to test the changes in the Pull Request.
[ ] I have detailed the functional tests required for approval.
[ ] I have performed a self-review of my code to ensure it is DRY and follows the team's coding standards.
[ ] I have commented my code, particularly in hard-to-understand areas.
[ ] I have made corresponding changes to the documentation.
[ ] I have verified that my code does not introduce a debug warning in my local environment.
[ ] I have verified that the functional tests work in my local environment.
[ ] I have verified that all automated tests pass or have provided a detailed comment about why I am submitting with a failed pipeline.
[ ] I have verified that any dependent changes have been merged and published in downstream commits.
[ ] I have added a link to this Pull Request in the Asana task.
[ ] I have moved the Asana task to "Ready for Functional Review".
[ ] I have left a comment in Asana and GitHub tagging a team member with a request for `review.

Testing

How to test the changes in this pull request

Follow the steps below to test the changes in this PR.

Deploy this branch to the prd test environment.
Navigate to https://kindlingprd.wpengine.com

Functional tests

As the functional tester for this pull request, I verify that:

[ ] The main navigation menu has a nicely styled mobile version.

Once testing is complete, notify the author of any failed tests and move the task to "Kick back" in Asana. If all tests pass, move the task to "Ready for Code Review" in Asana and tag a team member for code review.

Code review

As the code reviewer for this pull request, I verify that:

[ ] All automated tests have passed.
[ ] The code is written (or documented) in a way that is easy to understand.
[ ] The code is free of obvious errors.
[ ] The code is free of obvious duplication.
[ ] The code follows our coding standards.
[ ] The code is sanitized or escaped appropriately for any SQL or XSS injection possibilities.

Once testing is complete, notify the author of any failed tests and move the task to "Kick back" in Asana or continue with the "merging" steps below.

Merging

As the individual merging this pull request, I verify that:

[ ] All automated tests have passed.
[ ] All functional tests have passed.
[ ] All code review tests have passed.
[ ] I have moved the task to "Ready to Deploy" in Asana and notified the pull request author.

┆Issue is synchronized with this Asana task

matchboxdesigngroup / kindling