Add Dark/Light mode theming for Mermaid diagrams.

[rcannood]

Describe your changes

Add Dark/Light mode theming for Mermaid diagrams.

Issue ticket number and link

~Closes #xxxx~

Checklist before requesting a review

• [x] I have performed a self-review of my code

• Check the correct box. Does this PR contain:

  • [ ] Breaking changes
  • [ ] New functionality
  • [ ] Major changes
  • [x] Minor changes
  • [ ] Bug fixes

• [x] Proposed changes are described in the CHANGELOG.md

• [x] CI Preview succeeds and looks good!

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[rcannood]

Example: https://add-mermaid-theming--openproblems.netlify.app/documentation/reference/openproblems-v2/