Support dark theme on docs

[manuelpuyol]

Thanks @colebemis for the help!

This PR adds a way of toggling dark theme in the docs using the cmd + ctrl + t shortcut

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🔍 Inspect: https://vercel.com/primer/view-components/8nHLRkALwMtUziNZR94f4DrRoaKe
✅ Preview: https://view-components-git-docs-dark-theme-primer.vercel.app

[colebemis]

Woo! I'm glad this worked.

Might be nice to add some border to the Box:

[srt32]

👍 💯 I wonder if we could add an issue to track adding a UI element to toggle the theme. Discovering the keyboard shortcut is very not obvious IME

[manuelpuyol]

👍 💯 I wonder if we could add an issue to track adding a UI element to toggle the theme. Discovering the keyboard shortcut is very not obvious IME

I think that's intentional since it isn't fully shipped yet?
The color modes branch has a button to toggle it, so I think that when that's merged we can work on it too https://primer-css-git-mkt-color-modes-docs-primer.vercel.app/css/components/labels

[colebemis]

@srt32 From my PR in Primer React:

The keyboard shortcut is intentionally undiscoverable because it should only be used to visually test components as we migrate to functional color variables (part of the theming epic).
This is a temporary solution. When Primer React Components fully supports color modes, we'll be able to implement dark mode across the entire docs site instead of just the code examples.

primer/react#1095