Add dark mode theme support to documentation website

[GaneshPatil7517]

Summary

Add a dark mode toggle to the Apache Shiro documentation website to improve user experience and accessibility, especially for developers who prefer dark themes.

Current State

The website only supports a light theme. Many modern documentation sites (e.g., MDN, GitHub Docs) offer dark mode options.

Proposed Changes

1. Add CSS variables for theme colors in src/site/assets/css/base.css
2. Create dark mode stylesheet or CSS variables
3. Add a theme toggle button in the header template (src/site/templates/header.ftl)
4. Use JavaScript to persist user preference in localStorage
5. Respect system preference via prefers-color-scheme media query

Acceptance Criteria

• Dark mode toggle button visible in site header
• Theme persists across page navigation and browser sessions
• Respects OS-level dark mode preference as default
• Code blocks (Highlight.js) have appropriate dark theme
• All text remains readable with sufficient contrast (WCAG AA)
• No visual glitches during theme transition

Technical Notes

• Templates are in FreeMarker (.ftl files) in src/site/templates/
• CSS files are in src/site/assets/css/

Labels: enhancement, accessibility, css

[GaneshPatil7517]

hey @lprimak can i work on this...

[GaneshPatil7517]

hey @lprimak take loo at this when you get time: #277