Add 'system' theme option that follows OS dark/light mode

Feature Request

Add a "theme": "system" option that automatically switches the Claude Code theme based on the OS appearance setting (dark/light mode).

Current behavior

The theme is a static value (dark, light, light-daltonized, etc.). To auto-switch, users need to set up a SessionStart hook that checks defaults read -g AppleInterfaceStyle (macOS) and rewrites settings.json on every session start.

Desired behavior

A "theme": "system" setting that:

• Detects the OS appearance (dark/light) at session start
• Maps to a user-configurable pair, e.g.:

``json
{
"theme": "system",
"themeDark": "dark",
"themeLight": "light-daltonized"
}
``

• Ideally also reacts to appearance changes mid-session (macOS NSAppearance / Linux prefers-color-scheme)

Workaround

Currently using a SessionStart hook:

{
  "hooks": {
    "SessionStart": [{
      "hooks": [{
        "type": "command",
        "command": "if defaults read -g AppleInterfaceStyle &>/dev/null; then THEME=dark; else THEME=\"light-daltonized\"; fi; jq --arg t \"$THEME\" '.theme = $t' ~/.claude/settings.json > ~/.claude/settings.json.tmp && mv ~/.claude/settings.json.tmp ~/.claude/settings.json; echo '{\"suppressOutput\": true}'"
      }]
    }]
  }
}

This works but is fragile (rewrites settings.json, macOS-only, no mid-session switching).

Platform

macOS (but would benefit Linux/Windows users too)