[RFC] easier and more robust way to set theme for faency

[gndz07]

Description

Before v1.0.0-10, we could set theme for Faency via FaencyProvider quite easily, like so:

import { FaencyProvider } from '@traefiklabs/faency';

const App = () => <FaencyProvider primaryColor="$blue8">{/* your app */}</FaencyProvider>;

After this PR, this approach doesn't work anymore (the documentation inside readme is stale, cf https://github.com/traefik/faency/issues/402). The changes brought by #278 was motivated by the 'flashing' of the theme color because the selected theme was not available on the server side.

We could discuss on how to redesign this implementation and how to initialize theme so that it'll be easy to use by user.