feat: update higlight colors

[JTorreG]

Proposed changes

• Moves the highlight overrides to its own file to make it easier to make adjustments.
• Changes the override colors to use tones that pass the WebAIM contrast tests: WCAG AA and WCAG AAA
• Comply with DigitalA11Y requirements

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)