Display header in docs on mobile devices

[maebert]

Fixes #760.

Headers were not visible because of a transparent background. This commit will give the header the same gradient treatment as the sidebar, which will expose the hamburger menu - tapping the menu shows the sidebar for navigation.

Checklist

• The code change is tested and works locally.
• Tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?

[wren]

👍

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.