feat(poweredBy): add `theme` property

[shortcuts]

Summary

Similar implementation as in InstantSearch.js

• Adds a new theme prop to the poweredBy component to handle dark mode.
• Updates the Algolia logo

Result

Changes the logo/text color when theme="dark" is passed to the component

closes https://github.com/algolia/react-instantsearch/issues/3044

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routing-basic	Configuration

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[shortcuts]

Do we really need to change the API to support dark themes? Can't this be solved with CSS?

The API will be different from other InstantSearch flavors now.

This implementation is the same as in the InstantSearch.js PoweredBy component

[shortcuts]

It doesn't seem to work as expected on dark mode: See story.

@francoischalifour @Haroenv It seems like the issue is related to storybook not picking the changes locally.

Importing the component with import { PoweredBy } from '../packages/react-instantsearch-dom' in PoweredBy.stories.js actually show the changes.

https://user-images.githubusercontent.com/20689156/120993692-16f99d00-c784-11eb-9a84-135371e4928e.mov

[sarahdayan]

@shortcuts We're in the process of moving algolia/react-instantsearch to our InstantSearch monorepo. This PR won't be transferrable, so I'll close it here, but please feel free to reopen it once the migration is done.