Missing Dark Mode Support for amp.dev

### Description

The AMP website (https://amp.dev) does not support Dark Mode. Many modern websites provide dark mode for better accessibility and user experience.

### Alternatives Considered

- A dark mode option should be available for users.
- The website should automatically switch based on system preferences (`prefers-color-scheme`).
- Implement a CSS-based dark mode.
- Add a toggle button for manual switching.

### Additional Context

This issue is open for discussion and contributions.

The issue provides no files, tests, or entry points to follow. Start by reviewing amp.dev's current styling and the listed alternatives, then define the dark-mode behavior, including system preference and manual switching, before implementation; done means the site supports a usable dark theme.

Written by the indexing model from the issue text.