Add support for Dark Mode

## Feature Description

The [Dark Mode for WP Dashboard](https://wordpress.org/plugins/dark-mode-for-wp-dashboard/) plugin provides a nice dark theme for the WordPress dashboard.

Site Kit's WP dashboard widgets look good with the plugin enabled (although it would be nice to have a dark mode for the graph):

However, the Site Kit dashboard, of course, doesn't provide support for dark mode:

It would be great to add a dark mode theme, to help users reduce their eye strain and regulate their circadian rhythms when using the plugin.

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## Acceptance criteria

* A dark mode theme should be applied to the app automatically when the user's dark mode setting is set to dark mode.
* Dark mode should be detected in CSS using the `prefers-color-scheme: dark` media selector.
* The dark mode should be applied in SCSS *only* by overriding the primary colour variables for the plugin.

## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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