Update the default theme of simple_chat to make it look more interesting and highlight Flutter's theming support

_↴ Ported from [flutter/genui#563](https://github.com/flutter/genui/issues/563) — originally opened by [jacobsimionato](https://github.com/jacobsimionato) on 2025-11-19._
_Original labels: front-line-handled_

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Simple chat has that basic M3 look that everyone knows from hello_flutter. Let's add a custom theme, to give the impression that it can look interesting and you can customize it to your app's style.

Look at the simple_chat component's theme files, likely in a directory like `packages/simple_chat/lib/theme/`. Examine the current default theme to understand its structure. The goal is to create a more visually interesting custom theme that showcases Flutter's theming capabilities. Check for existing examples of custom themes in the project to follow the same patterns. Test the changes by running the simple_chat example to see the new appearance.

Rédigé par le modèle d'indexation à partir du texte de l'issue.