content/static: add CSS styling to the new embedded playground
content/static: add CSS styling to the new embedded playground

This CL adds the CSS styling for the new playground examples. It is
currently under a feature flag, so the CL won't make any visible changes to the
website yet.

Without error: https://i.imgur.com/OK4V0sU.png
With error: https://i.imgur.com/sfqhZub.png

For golang/go#36865

Change-Id: I8d0e2aeeb4076bb863d36887bfe8cb0d791a1f25
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/243638
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite