The shared `copilotkit-overrides.css` files in langgraph-python,
langgraph-typescript, mastra, built-in-agent, and the starter template
forced `.copilotKitChat { background-color: #fff !important; }`, which
won over the demo-level `ThemeProvider` and made the beautiful-chat
demo render a white panel in dark mode. langgraph-fastapi has no
overrides file and was unaffected — same fix gets the others to parity.
Also add a warning in showcase/STYLING-GUIDE.md so the example block
isn't pasted back in by the next contributor.
Alem Tuzlak