fix: improve back-to-top button contrast

[saurabhhhcodes]

Summary

• replace the hard black back-to-top button styling with a higher-contrast cyan treatment
• add hover and focus-visible states that stay readable in both light and dark themes
• add a tiny regression test that pins the theme-safe button classes

Validation

• python3 -m pytest tests/test_back_to_top_styles.py -q
• git diff --check

[Aditya948351]

Please link the issue here that this PR of yours fixes

[Aditya948351]

Resolved conflicts and merged manually via #341. Thank you!