Allow customisation of user message background colour in conversation view

Feature Request

What: Allow users to customise the background highlight colour behind their sent messages in the Claude Code conversation view.

Current behaviour: User messages are displayed with a hardcoded #373737 background. This colour cannot be changed through any configuration — not via settings.json, Ghostty palette, or tmux settings.

Why it matters: The user message background is the primary visual anchor for scanning back through a conversation to find where you last spoke. A configurable colour would let users:

• Pick a colour with better contrast against their terminal theme
• Match their personal colour scheme / aesthetic
• Improve readability for accessibility needs

Suggested approach: A theme key in ~/.claude/settings.json, e.g.:

{
  "theme": {
    "userMessageBackground": "#2d1f3d"
  }
}

Alternatively, mapping it to an ANSI palette index so terminal theme authors can control it via their palette would also work.

Environment: macOS, Ghostty terminal, tmux, Claude Code CLI.