🐛 Bug: Audit and fix accessibility for browser reporter

[Josh Goldberg ✨]

Bug Report Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I have 'smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, my usage of Mocha, or Mocha itself.
• I want to provide a PR to resolve this

Expected

There should be no web accessibility issues with the browser reporter.

Actual

A quick axe DevTools scan reveals 3 Elements must meet minimum color contrast ratio thresholds issues.

Minimal, Reproducible Example

See index.html from mochajs/mocha-examples#72.

Versions

Mocha 10.4.0.

Additional Info

This has been an issue for a while. I wanted to wait until after #5113 to bring it up, so as to not get in the way of that PR. 🙂