Dark mode 🌙

[austincondiff]

Based on the design in #2557, and leveraging our existing theme, create a new theme that uses dark colors to support dark mode. I will be updating this CodeSandbox with a demo of how this might work.

Note: We should default to use the users preferences using the new prefers-color-scheme.
I.e. window.matchMedia("(prefers-color-scheme: dark)").matches

• Add dark theme
• Test code
• Open a pull request

[reimertz]

This is awesome work! I love it!