docs: document dark mode and theme toggle

[SemTiOne]

Description

Adds docs/user/appearance.md documenting Osprey's theme support, how it follows OS/browser preference by default, and how to manually override it via the toggle in the navigation bar.

Closes #384

• docs/user/appearance.md — new page
• docs/SUMMARY.md — add Appearance to the User Guide sidebar
• docs/user/README.md — link to appearance.md

Note: screenshots to be added, requires a running Osprey instance.

Checklist

• Tests pass locally
• uv run ruff check . passes (no unused imports or other lint errors)
• uv tool run fawltydeps --check-unused --pyenv .venv passes (no unused dependencies)
• Updated CHANGELOG.md with my changes, if applicable

Summary by CodeRabbit

• Documentation
  • Added a new user guide page covering light, dark, and system theme options.
  • Documented that the selected theme is saved between sessions.
  • Noted that system theme mode updates automatically when the device’s color scheme changes.
  • Updated the main user documentation to mention theme support and link to the new page.
  • Added the new page to the documentation table of contents.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: eb1afb9d-9b7e-4878-9768-0e6b4f31a8f7

📥 Commits

Reviewing files that changed from the base of the PR and between fa17a2a and 789eeb5.

📒 Files selected for processing (3)

• docs/SUMMARY.md
• docs/user/README.md
• docs/user/appearance.md

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📝 Walkthrough

Walkthrough

Adds a new docs/user/appearance.md page documenting Osprey's light, dark, and system theme options, including persistence and real-time OS preference updates. Registers the page in docs/SUMMARY.md and adds a brief reference in the user guide README.

Changes

Appearance documentation

Layer / File(s)	Summary
Appearance guide, TOC entry, and README reference docs/user/appearance.md, docs/SUMMARY.md, docs/user/README.md	New page describing theme toggle options (Light, System, Dark), browser-storage persistence, and real-time system preference updates; linked from the TOC and the user guide intro.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: documenting dark mode and the theme toggle.
Linked Issues check	✅ Passed	The new appearance guide covers light/dark/system theming and the manual override behavior requested by #384.
Out of Scope Changes check	✅ Passed	All changes are documentation updates directly tied to the dark mode theme guide.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[cassidyjames]

Thanks for the contribution! Let's get a screenshot in here and then I think it's good; I'm not sure if we need a dedicated page, but we can always move it later.

[cassidyjames]

I'm happy to merge this as-is, and then we can follow up with screenshots or moving things around if we want. Thanks again @SemTiOne!