[codex] Add dark mode to documentation site

[Brett-Best]

Summary

• Adds system dark mode support to the documentation site via CSS custom properties and prefers-color-scheme.
• Updates generated Rouge syntax CSS to include both github.light and github.dark themes.

Notes

• Consulted CONTRIBUTING.md for branch/PR conventions and site/README.md for the Jekyll site workflow.
• The workspace AGENTS.md says PRs should target develop, so this PR uses develop as the base branch.

Validation

• ruby -c site/site_content.rb
• git diff --check
• RBENV_VERSION=3.4.1 BUNDLE_PATH=/private/tmp/swiftformat-bundle rbenv exec bundle exec rake site:build
• Captured screenshot evidence for both impacted site pages in light/dark mode at desktop and mobile widths.

Screenshots

Home

Desktop light	Desktop dark

Mobile light	Mobile dark

Rules

Desktop light	Desktop dark

Mobile light	Mobile dark

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.38%. Comparing base (92882ca) to head (0f316e1).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2586   +/-   ##
========================================
  Coverage    95.38%   95.38%           
========================================
  Files          176      176           
  Lines        26193    26193           
========================================
+ Hits         24983    24985    +2     
+ Misses        1210     1208    -2     

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[Brett-Best]

@calda the website looks good however it was very bright! Here is a vibe coded dark mode 🙏🏻, looks reasonable 🤔

[Copilot]

Pull request overview

Adds automatic (system) dark mode support for the SwiftFormat documentation site by introducing a small design-token layer (CSS custom properties) and generating a dark-mode Rouge syntax highlighting stylesheet gated by prefers-color-scheme.

Changes:

• Introduces color custom properties in :root, with dark-mode overrides under @media (prefers-color-scheme: dark), and updates existing selectors to use them.
• Updates site generation to produce a single syntax.css that contains both github.light and github.dark Rouge themes, applying the dark theme via prefers-color-scheme.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
site/src/assets/css/style.css	Adds theme variables + dark-mode overrides and migrates existing hardcoded colors to variables for consistent light/dark rendering.
site/site_content.rb	Generates combined light+dark Rouge syntax CSS, wrapping the dark theme output in a prefers-color-scheme: dark media query.

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[calda]

Thank you!

I tweaked the theming a bit to be slightly less black, similar to the github.com light dark theme (more similar to macOS dark mode).