Allow customizing user-message background color in TUI

What I'd like

Right now user messages render with a subtle grey background block, which blends into the conversation in long sessions. Would love a way to make my own messages stand out — e.g. an orange/highlighted background so I can scan back through a long stack and find where I last interjected.

Suggested surface

• A new theme.userMessageBackground (or similar) field in settings.json, accepting a color name or hex.
• Or a richer theme system where the existing /config → Theme picker exposes per-element colors (user msg bg, assistant msg bg, diff bg, tool-call bg, etc.).

Use case

I run very long autonomous sessions and visually re-locating my own interjections in scrollback is harder than it should be. Coloring just the user-message background would solve it without affecting any other rendering, and wouldn't require a full theming overhaul.

Today's workarounds (all unsatisfying)

• Terminal-level pattern triggers (iTerm2 / wezterm) — fragile, depends on Claude Code's exact rendering.
• /config → Theme — only switches dark/light/colorblind palettes; doesn't target user messages specifically.
• tui setting — layout only, no color hooks.