Add Dark Mode and Theme Toggle Support to Trac

[kehach07]

Summary

This draft PR introduces dark mode support and theme toggle functionality to Django's Trac instance (code.djangoproject.com), aligning its design, color tokens, and toggle behavior with the main djangoproject.com website.

Work done under Google Summer of Code (GSoC) 2026 with the Django Software Foundation.

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Key Changes

• Theme Toggle Component: Implemented a 3-state switcher (Auto $\rightarrow$ Light $\rightarrow$ Dark) matching the upstream toggle behavior on the main Django website.
• Template Updates: Updated trac-env/templates/site_header.html and site_head.html to inject theme markup, include accessibility hooks, and load dark mode scripts.
• Stylesheets & Tokens: Created scss/trachacks.scss and darkmode.css with clean CSS custom properties for surfaces, borders, ticket tables, query builders, and admin views.
• Accessibility Enhancements: Added accessible labels (aria-label, visible focus outlines, and compliant .visually-hidden styling) and fixed contrast ratios across key pages.
• Dev Workflow & Docs: Updated README.rst and docker-entrypoint.sh for smoother static asset builds and local plugin registration checks.

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Testing & Verification

• Verified theme switching and state persistence across page refreshes.
• Confirmed support for system preference detection (prefers-color-scheme).
• Tested keyboard accessibility and screen-reader announcements using Accessibility Insights for Web.
• Verified views locally across /query, /ticket, and /admin.

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Related Links & References

• Issue / Upstream Discussion: djangoproject.com #2650
• Development PR / Branch: Add Dark Mode and Theme Toggle Support sabderemane/code.djangoproject.com#1