Proposal: optional dark theme + toggle for the examples

Would you be open to adding a dark theme + toggle to the examples?

The blog post frames these as throwaway artifacts you prompt from scratch, but in practice I've found the repo to be a nice canonical base to derive project/company-specific variants from. A built-in dark mode would make that even more convenient for anyone consuming this repo.

What I had in mind:
- A small inline CSS-variable layer over the existing brand palette, plus a fixed toggle button. Self-contained per file, with no build step or external dependencies.
- Honors the OS `prefers-color-scheme` but remembers the user's manual choice. Light stays the default.
- Leaves SVG/illustration content alone, since whether a given figure should be light or dark is context-specific.
- Presentation-only: no changes to the example content or the index.html structure.

Totally fine if you feel that things like this should live downstream. It's already prototyped, so I'll follow up with a draft PR to illustrate.