Style the Search Bar in Dark Mode

[iamrecursion]

General Summary

Currently when you swap to dark mode the search bar stays bright. It should invert colours like the rest of the theme does.

Motivation

A coherent dark mode.

[joenash]

(note to self)
Will require adding the darkmode styles to the widget in algolia.html via https://www.algolia.com/doc/guides/building-search-ui/widgets/customize-an-existing-widget/js/

Algolia

Customize an existing widget

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