[FEATURE] VS Code extension: make user-message blocks visually distinguishable (themable color / configurable style)

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

In the native VS Code UI, my prompts and Claude's responses read as one continuous stream. .userMessage uses --app-input-background + --app-input-border, which in most themes is nearly identical to the surrounding panel background, so when scrolling back through a long conversation I can't quickly find where each of my turns starts.

Proposed Solution

Either of:

Register the chat colors via contributes.colors (e.g. claudeCode.userMessageBackground, claudeCode.userMessageBorder, claudeCode.userMessageForeground) so they can be overridden in workbench.colorCustomizations per theme. Preferred — costs nothing by default and lets every theme author tune it.
Or raise the default contrast of the user-message block (distinct background tint, or a left accent bar), plus a claudeCode.userMessageStyle setting (subtle | bordered | accent).

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

Version: extension 2.1.228 (darwin-arm64), CLI 2.1.219, macOS 15/Darwin 25.3.