[FEATURE] Disable or Customize User Input Background Highlighting

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

Problem
Claude Code applies a white background highlight to user input in the terminal, which makes text unreadable for users with bright-colored terminal themes (e.g., bright green text on black background).

### Current Behavior

• User messages are displayed with a white/light background highlight
• This highlighting cannot be disabled or customized through:
• Output styles (only affects response formatting, not UI rendering)
• ~/.claude/settings.json
• Environment variables
• Command-line flags

Proposed Solution

### Requested Solution
Add configuration options to control user input highlighting:

Option 1: Disable highlighting entirely
```json
{
"ui": {
"highlightUserInput": false
}
}

Option 2: Customizable highlight colors
{
"ui": {
"userInputBackground": "none",
"userInputBorder": "blue"
}
}

Use Case

Users with custom terminal themes (bright colors, high contrast, accessibility themes) need readable text. Current white background makes green/yellow/cyan text completely unreadable.

Alternative Solutions

Workarounds Attempted

• Created custom output style - doesn't affect UI rendering
• Checked settings.json - no relevant options
• Reviewed documentation - no color customization available

Priority

Critical - Blocking my work

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_