Add keyboard shortcut to preview live examples in dark mode

[colebemis]

This PR adds a keyboard shortcut (ctrl + cmd + T) to preview live examples in dark mode. 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.

Note: Some components might not work in dark mode (even if they use functional variables) until the default theme prop is removed (PR: #1094)

[changeset-bot]

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Latest commit: a53239f

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[bscofield]

😍 Thank you!