Support dark mode styling in docs and marketing sites

[astrobot-houston]

Originally proposed by @NuroDev in #550. Their description is reproduced verbatim below.

All community-submitted pull requests are automatically converted to issues (bugs) & discussions (feature requests, enhancements) where they can be triaged and prioritized. Once prioritized, a PR implementation is created automatically.

Warning

Experimental! Do NOT merge yet.

Summary

Makes it a bit easier on the eyes when reading the docs

• Add dark mode support across the documentation and marketing apps.
• Update shared Astro components, layouts, navigation, cards, and site styling for dark themes.
• Configure both apps to support the new theme consistently.

Testing

Not run (not requested)

Screenshots

---

Original implementation from #550 by @NuroDev