Sidebar improvements

[lamATnginx]

Proposed changes

• Changing font weight on sidebar to be thinner.
• Refactored sidebar lines to be centered.
• Converted from rem to px.
• Changed the JS for the site dropdown to remove border at the bottom when dropdown is active.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[ADubhlaoich]

Curious about the change from relative units to fixed units, but I trust it's for good reason.