Add 'system' theme option to follow OS appearance

Summary

Claude Code's theme setting offers light and dark modes, but no "system" option that follows the OS appearance. On macOS (and other platforms), the OS can automatically switch between light and dark mode based on time of day. Claude Code should be able to follow this setting.

Proposed behavior

Add a "system" option to the theme setting that tracks the OS appearance mode at startup (and ideally reacts to changes during a session).

On macOS, the current appearance can be read via:

defaults read -g AppleInterfaceStyle 2>/dev/null  # returns "Dark" or fails (= Light)

Motivation

Users whose OS switches appearance based on time of day currently have to manually toggle the Claude Code theme via /config each time. A "system" option would make this seamless.