feat(carousel): introduce styling for all themes

[aymeric-giraudet]

Summary

FX-2966

Result

Ultimately chose to put defaults in reset.scss, it may be sort of big relatively to the file but takes up less space than the range slider, and it's still small relative to the overall size of InstantSearch.

• reset sets defaults for the carousel, it should be a grid with a horizontal scrollbar and centered buttons on each side on hover. Navigation buttons are unstyled.
• Each theme then styles the buttons, and sets spacing and colors. I only used colors from their respective palettes.

(I added red background and height to each item to make gaps and size obvious) reset:

algolia:

satellite:

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Sandbox	Source
example-instantsearch-getting-started	Configuration
example-react-instantsearch-getting-started	Configuration
example-react-instantsearch-next-app-dir-example	Configuration
example-react-instantsearch-next-routing-example	Configuration
example-vue-instantsearch-getting-started	Configuration