Adjust colors of out-of-the box catalog to look good in both light and dark default flutter theme.

_↴ Ported from [flutter/genui#745](https://github.com/flutter/genui/issues/745) — originally opened by [polina-c](https://github.com/polina-c) on 2026-02-21._
_Original labels: P2, front-line-handled, a: api-simplicity_

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1. Add tabs to catalog gallery to show different options:
light / dark, scaffold / card / body background.

2. Work with UX to make sure colors of all catalog items look good with all options

Issue example:

The issue involves the catalog gallery in the a2ui project. First, locate the catalog component files, likely in a directory like 'catalog' or 'gallery'. Examine how colors are currently applied and where theme switching logic exists. The task is to add tabs for light/dark and scaffold/card/body background options and adjust colors for all catalog items to look good in both themes. Work with UX as mentioned. Check for existing theme utilities and color definitions. Verify changes by running the catalog app and toggling themes.

索引モデルが issue の本文から書いたものです。