Add dark mode style color for contrast in docs

[d33bs]

Description

This PR addresses #428 by allowing the documentation to use distinct colors for styling in light and dark modes. While I made these changes I also slightly reorganized the comments and code nearby for the Sphinx theme configuration.

Additionally, I noticed Read the Docs builds were failing, seemingly due to the environment not installing properly from Poetry. I added a small fix to .readthedocs.yml to try and re-enable builds which seems to have worked.

I manually tested the web accessibility of the color contrast using Lighthouse through the developer console in Chrome. Let me know if you think we should update the color to a different one based on the contrast or how it aligns with other styling.

Closes #428

What is the nature of your change?

• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• This change requires a documentation update.

Checklist

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• I have read the CONTRIBUTING.md guidelines.
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📚 Documentation preview 📚: https://pycytominer--429.org.readthedocs.build/en/429/

[kenibrewer]

LGTM!

[d33bs]

Thanks @kenibrewer ! Going to merge this in.