Implement dark mode via light-dark() fn

[Turbo87]

This implements a dark mode for the user interface, based on the system theme setting. The color-scheme CSS attribute could eventually be overridden if the user does not want to use the system setting.

Examples

[Turbo87]

looks like I might have to double check that the PostCSS plugin is working correctly (see Percy), but this should still be available for review. I hope I didn't forget any pages...

also, do we require a user override button to ship this or can we ship this without one?

[eth3lbert]

I would expect a color scheme toggle button to be included with this PR. This would allow users who don't want this change to simply switch back to the light theme.

[bors]

☔ The latest upstream changes (presumably 16b9eab) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably 748e265) made this pull request unmergeable. Please resolve the merge conflicts.

[eth3lbert]

I've implemented a simple dark mode toggle that can switch between light, dark, and system. Feel free to cherry-pick or modify it as needed if you find it helpful in any way. eth3lbert@0dc59e4

[Turbo87]

@eth3lbert thanks! I've cherry-picked the UI from your commit, but simplified the implementation a little bit :)

edit: I've also set the default to the "light" color scheme for now in case we find any issues with the dark mode.

[eth3lbert]

That's great! Adding an icon as a visual cue to the dropdown menu is a much better implementation. I love this! I've tested it locally on the landing page without any issues.