feat: Implement Dark and Light mode support with topbar toggle and auto-detection

[O2sa]

Overview

This PR adds full Dark & Light Mode support across oauth.net, allowing users to seamlessly switch themes via a topbar button while respecting system OS preferences by default.

What's Changed

1. Theme Variables & Styling (public/stylesheets/style.css)

• Extended the CSS custom properties system with [data-theme="dark"] and @media (prefers-color-scheme: dark):
  • Deep slate background (#0b1120) and surface container (#151f32) palettes.
  • High-contrast text colors (#e2e8f0 / #94a3b8) and accessible link/heading accents (#93c5fd / #38bdf8).
  • Dark mode support for inline code, RFC badges (.rfc-badge), spec status pills, decision tree cards, topic links, and table borders.
  • Added styling and transitions for the .theme-toggle button.

2. Zero-Flicker Script, Version Bump & Header Switcher (includes/_header.php)

• Incremented style.css cache-busting version to ?9.
• Added a render-blocking inline script in <head> that reads localStorage.getItem('theme') or window.matchMedia('(prefers-color-scheme: dark)') to set data-theme immediately before DOM paint (preventing FOUC).
• Added an accessible Sun ☀️ / Moon 🌙 SVG toggle button inside .site-topbar.

3. Event Handling & OS Synchronization (includes/_footer.php)

• Added JavaScript event listener for #theme-toggle to cycle between light and dark modes and persist the user's choice in localStorage.
• Added listener for OS prefers-color-scheme changes when no manual override is stored.

4. Documentation (REDESIGN.md)

• Updated REDESIGN.md to move "Dark mode" from "Future Considerations" to "Completed".