Add color/theme customization for CLI UI elements

Problem

Claude Code's interactive UI elements (yes/no prompts, selection highlights, permission dialogs) use hardcoded 24-bit RGB colors via true color escape sequences. These colors bypass the terminal's 16-color ANSI palette entirely, so users cannot adjust them through their terminal theme.

This is an accessibility issue for users with color vision deficiencies. For example, with slight red/green color weakness, the blue highlight on selected options in yes/no prompts is difficult to distinguish from unselected options.

Evidence

• Setting ANSI palette colors 4 (blue) and 12 (bright blue) to completely different colors (hot pink, green) in a Ghostty terminal theme has zero effect on Claude Code's UI blues.
• The same palette changes correctly affect other terminal programs (ls, git, etc.) that use standard ANSI colors.
• ghostty +show-config confirms the terminal is loading the custom palette values.
• The Claude Code binary contains hardcoded rgb(R,G,B) values that get emitted as \x1B[48;2;R;G;Bm sequences.

Requested Feature

A way to customize Claude Code's UI colors — any of these would work:

1. Use ANSI palette colors instead of hardcoded RGB for UI elements, so terminal themes can control them
2. A config option (e.g., claude config set theme.selectionBackground "#RRGGBB")
3. A high-contrast mode flag for users with color vision deficiencies
4. An environment variable to override the color scheme

Environment

• Claude Code 2.1.76
• Terminal: Ghostty (true color supported)
• OS: Arch Linux