feat: Add Dark Mode Support to Ionicons Website

### Describe Problem

Currently, the Ionicons website does not support dark mode. Many users prefer or rely on dark themes to reduce eye strain, especially in low-light environments or at night. Having only a light mode can make it harder for some users to navigate the site comfortably.

### Describe Preferred Solution

- Implement a CSS-based dark mode.
- Add a toggle switch to enable or disable dark mode manually or automatically.
- Ensure Ionicons Web Components have proper contrast in dark mode.
- Match the dark mode experience with other Ionic ecosystem sites.

### Describe Alternatives

_No response_

### Additional Information

_No response_

The issue names no files, tests, or entry points. First locate the Ionicons website styles and the UI entry point for theme selection; done means CSS dark mode, a manual or automatic toggle, readable Ionicons Web Components, and visual consistency with other Ionic sites.

Scritto dal modello di indicizzazione a partire dal testo della issue.