[Bug] Diff view text unreadable on light terminal backgrounds without theme configuration

Bug Description
Title: Diff view text unreadable on light terminal backgrounds (no theme configured)

Description:

When Claude Code displays a diff (edit permission prompt) in a terminal with a white/light
background and no theme explicitly configured, the highlighted changed lines become unreadable.

Steps to reproduce:

1. Use macOS Terminal with the default white background
2. Have no theme set in ~/.claude/settings.json
3. Trigger a file edit that requires permission (default permission mode)
4. Observe the diff view shown before confirming the edit

Expected behavior: Changed lines are clearly readable regardless of terminal background color.

Actual behavior: The added line (green highlight) renders with dark/black text on a dark green
background, making it illegible. The surrounding context lines also render with very low contrast
against the white background.

Environment:

• macOS Terminal (default theme, white background)
• No theme key in ~/.claude/settings.json
• Platform: macOS

Root cause hypothesis: The diff highlight colors appear hardcoded for dark terminal backgrounds
and do not adapt to light backgrounds or respect the terminal's color scheme./Users/potato/Desktop/Screenshot\ 2026-05-16\ at\ 18.39.31.png

Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.143
• Feedback ID: 0d784216-3282-4de4-8f9c-1d6dfdabf5b7

Errors

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Screnshot

<img width="1220" height="491" alt="Image" src="https://github.com/user-attachments/assets/e3c54c90-e3fb-43b0-b172-17323c109e26" />