fix: visual QA on SiteHeader

[emilyjablonski]

Pull Request Template

Issue

Addresses #1345

• This change addresses the issue in full
• This change addresses only certain aspects of the issue
• This change is a dependency for another issue
• This change has a dependency from another issue

Description

Three visual issues in the ticket.

Would appreciate a lil extra testing around the hover pixel shift in partners on "sign out" since I couldn't get this to happen on my machine but I think I fixed it anyway!

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Prototype/POC (not to merge)
• This change is a refactor/addresses technical debt
• This change requires a documentation update
• This change requires a SQL Script

How Can This Be Tested/Reviewed?

Check out the issue and test the 3 bugs.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have reviewed the changes in a desktop view
• I have reviewed the changes in a mobile view
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have assigned reviewers
• I have updated the changelog to include a description of my changes
• I have run yarn generate:client if I made backend changes
• I have exported any new pieces in ui-components

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[slowbot]

@emilyjablonski LGTM