Allow customizing user message background color in the UI

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

When chatting in Claude Code, it's hard to visually distinguish my own messages
from Claude's responses at a glance. The current interface only supports
dark/light theme switching, with no way to customize individual message colors.

Proposed Solution

Add an option in settings to customize the background color of user messages
(e.g., light yellow, light blue). This would make it much easier to visually
scan the conversation.

Alternative Solutions

_No response_

Priority

Critical - Blocking my work

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_