Allow customizing message colors for user vs assistant in VSCode extension chat panel

Feature request

In the VSCode native extension chat panel, there is currently no way to visually distinguish messages from the user vs. messages from the assistant using different colors.

Current behavior:

• The extension settings (useTerminal, preferredLocation, hideOnboarding) don't expose any color/theme customization.
• The chat panel appears to be an internal webview, so VSCode's workbench.colorCustomizations does not reach it.
• Switching to terminal mode (useTerminal: true) doesn't help either — no per-message-type color customization there.

Request:
Add a setting (or theme token) that lets users assign different text/background colors to user messages vs. assistant messages in the chat panel, to make longer conversations easier to scan at a glance.

Thanks!