🎨 Fix styling of docs headings in dark mode

[mermop]

This is a very minor change to the CSS in the docs to make it a little nicer to read in dark mode.
This doesn't close any issues and I wouldn't expect it to need any test or docs updates.

• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

Before

After

[codspeed-hq]

CodSpeed Performance Report

Merging #2026 will not alter performance

_{Comparing mermop:patch-1 (6959193) with master (ad02b87)}

Summary

✅ 437 untouched benchmarks