Apply PaperMod-inspired styling and theme toggle

[magic-alt]

Summary

• restyle the layout with PaperMod-inspired surfaces, typography, and card treatments across the home, archive, tag, and about pages
• add a persistent light/dark mode toggle that honours system preference and updates the browser theme color
• streamline head/footer includes to initialise the theme early and load MathJax assets only when required

Testing

• bundle exec jekyll build (fails: bundler: command not found: jekyll)
• bundle install (fails: Gem::Net::HTTPClientException 403 "Forbidden")

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https://chatgpt.com/codex/tasks/task_e_68f1c75993848328908df9a8c66dcbd5

[Copilot]

Pull Request Overview

This PR implements a complete design system overhaul with PaperMod-inspired styling, introduces a functional light/dark theme toggle, and optimizes asset loading. The changes transform the site from a single-theme layout to a modern, accessible design system with proper theme management.

• Comprehensive CSS rewrite with new design tokens and modern card-based layouts
• Interactive theme toggle with system preference detection and persistent storage
• Conditional MathJax loading and streamlined head/footer structure

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
assets/css/styles.css	Complete styling overhaul with design tokens, theme variables, and modern layout components
_includes/head.html	Added theme initialization script, conditional MathJax loading, and theme-aware meta tags
_includes/footer.html	Implemented theme toggle functionality and removed duplicate MathJax script

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[Copilot]

Hard-coded color value should use a CSS custom property instead. Consider using var(--color-accent-hover) or similar for consistency with the design token system.

[Copilot]

Hard-coded color value should use a CSS custom property. Consider defining --color-white: #ffffff in the root for consistency.

[Copilot]

Hard-coded color value should use a CSS custom property. Consider using the same --color-white variable suggested for other white color instances.

[Copilot]

Hard-coded color values should reference the CSS custom properties defined in styles.css. Consider using the same values as --color-bg for consistency.