[FEATURE] Auto-detect terminal background to keep diffs readable

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When the system theme changes (light → dark or vice versa), terminals like Ghostty automatically follow. However, Claude Code keeps its previous theme, making diff highlights and syntax coloring unreadable — dark-on-dark or light-on-light. The only workaround is manually running /config every time.

Proposed Solution

Add a theme: "auto" option that queries the terminal's background color using the OSC 11 escape sequence (\e]11;?\a) and selects light/dark accordingly. This should run on startup and periodically or on resume to catch mid-session changes. Most modern terminals support OSC 11: Ghostty, iTerm2, Kitty, Alacritty, WezTerm, Windows Terminal.

Alternative Solutions

A hook-based approach where users wire up their own detection script, but this is complex for a common use case.

Priority

Medium

Feature Category

Configuration and settings

Use Case Example

I use Ghostty which follows macOS/Linux system appearance. When I switch to dark mode in the evening, Ghostty updates instantly but Claude Code's diffs become illegible until I notice and manually run /config.

Additional Context

OSC 11 is a well-established standard — no terminal-specific integration needed.