Feature request: Add Appearance toggle (System / Light / Dark) in Settings → Appearance

[ds2606]

Hi 👋 Thanks for Runestone — it’s a fantastic editor.

In the app, under Settings → Appearance, the editor’s themes
are currently tied to the system appearance.

Feature request: add an explicit Appearance toggle for the editor, with three modes:

• System
• Light
• Dark

One possible UI is a segmented control at the top of the Appearance screen, where the chosen value is:

• Persisted in UserDefaults
• Applied via the existing theme system / UI style

I’d be happy to work on a PR implementing this if the approach seems aligned with the project’s design goals.

Thanks for considering!