fix(theme): improve theme color contrast for macOS Terminal.app

[clocky]

What happened?

In macOS Terminal.app and other xterm-256 compatible environments, certain default theme colors (both light and dark) have insufficient contrast, making text difficult to read. Specifically, some accent colors fall below the recommended 4.5:1 contrast ratio against the default backgrounds.

What did you expect to happen?

All theme colors should maintain a minimum 4.5:1 contrast ratio for accessibility and clear legibility across all supported terminal emulators, including macOS Terminal.app.

Client information

Client Information

Platform: macOS
Terminal: macOS Terminal.app (xterm-256color)

Login information

N/A (Theme-related UI issue)

Anything else we need to know?

Fixed by PR #21169. This issue specifically targets accessibility improvements for the default themes in the terminal.

[gemini-cli]

Found possible duplicate issues:

• text color is unreadable if not in dark mode. please make color contrast better for light mode #12961
• Viewing dark themes in light terminals are unreadable #4611

If you believe this is not a duplicate, please remove the status/possible-duplicate label.