Desktop app: allow custom styling of chat message bubbles (e.g. user-stylesheet or theme colors)

Feature request

Area: Claude desktop app (Code tab)

What I want

A way to tune the look of the chat window, specifically the background color of my own message bubbles. For example, a light green background on the user's messages so they stand out from Claude's replies when scanning a long session.

Suggested shapes (any one would do)

• A per-user custom stylesheet (CSS) hook for the desktop app, similar to what VS Code and other Electron apps allow via extensions or a user-styles file.
• Theme settings exposing a few colors (user bubble background, assistant bubble background, accent) in Settings > Appearance.
• A per-session override via the existing session title-bar menu, alongside Output style.

Why

Long coding sessions interleave many short user prompts with long Claude responses. A distinct color on my own messages makes it much faster to find where I asked something. Today the only appearance controls are the built-in light/dark themes, and there is no CSS or stylesheet setting in the app's config directory.

Environment

• macOS (Darwin 25.6)
• Claude desktop app, Code tab