Add system theme support (auto-switch between light/dark)

[IrumiDesu7]

Description

Claude Code should support following the system theme, automatically switching between light and dark modes based on OS preferences.

Current Behavior

Claude Code has theme settings accessible via /theme or /config commands, allowing users to manually select between light and dark themes. However, there is no option to automatically follow the system theme.

Desired Behavior

Similar to VS Code's system theme support, Claude Code should automatically switch themes based on OS preferences.

VS Code implements this with settings like:

{
  "window.autoDetectColorScheme": true,
  "workbench.preferredDarkColorTheme": "Solarized Dark",
  "workbench.preferredLightColorTheme": "Solarized Light"
}

Claude Code should offer similar functionality:

• Automatically switch to dark theme when system is in dark mode
• Automatically switch to light theme when system is in light mode
• Update in real-time when system theme changes (no restart required)

Use Case

Users who switch between light mode (daytime) and dark mode (nighttime) at the OS level expect all their development tools to follow the same preference. This creates a consistent experience across VS Code, terminals, and Claude Code.

Proposed Implementation

Add a new theme option alongside existing light/dark themes:

Via /theme command:

• light (existing)
• dark (existing)
• auto or system (new - follow OS theme)

Or in ~/.claude/settings.json:

{
  "theme": "auto"
}

Environment

• OS: macOS (though this should work on all platforms)