Enhance the retro theme's API and documentation pages to better mimic
the Windows 98 aesthetic:
- Convert the docs header into a full-width grey window with black text.
- Style section card headings (`h2`) as classic blue gradient title bars, complete with a mock close button ("✕").
- Adjust margins to make top-level headings flush with window edges.
- Hide the kicker element in the docs header.
Add a new "retro" theme option that mimics the classic Windows 98 aesthetic, providing a nostalgic alternative to the modern and classic dark themes.
Changes include:
- Defining CSS variables for the retro theme in `00-base.css` (pixel fonts, silver/gray colors, and classic window shadows).
- Adding custom styling for cards, headers, buttons, and title bars to replicate classic OS windows.
- Adding a star background GIF (`stars1.gif`).
- Excluding the retro theme from modern "revamp" styles in `15-revamp.css`.
- Updating `CLAUDE.md` with instructions on screenshot verification.
