[FEATURE] Allow customization of user message background color in terminal

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

After sending a message in Claude Code, the user's prompt is displayed with a dimmed
yellow background highlight. When scrolling back through a long conversation to find
previous prompts, this dimmed yellow is very hard to distinguish from the rest of the
output — especially on dark terminal backgrounds.

User messages should be the easiest thing to spot when scrolling, since they mark the
start of each interaction. Currently they blend in.

Proposed Solution

Allow users to customize the user message background color via settings.json, e.g.:

```json
{
"theme": {
"userMessageBackground": "brightYellow"
}
}

  Or at minimum, provide a few built-in theme presets (e.g., high-contrast, dark, light)
  that adjust the user message styling for better visibility.

### Alternative Solutions

_No response_

### Priority

Medium - Would be very helpful

### Feature Category

Interactive mode (TUI)

### Use Case Example

_No response_

### Additional Context

_No response_