Issue-1449: Fix Post header overlap (z-index) issue with hamburger menu.

[tianlangwu]

Issue This PR Addresses

Fixes #1449

Type of Change

• Bugfix: Change which fixes an issue
• New Feature: Change which adds functionality
• Documentation Update: Change which improves documentation
• UI: Change which improves UI

Description

decrease the z-index of the header, and make it lower than z-index of the hamburger menu.

Checklist

• Quality: This PR builds and passes our npm test and works locally
• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not (if applicable)
• Documentation: This PR includes updated/added documentation to user exposed functionality or configuration variables are added/changed or an explanation of why it does not(if applicable)

[humphd]

Nice!