_content/css: improve contrast for playground and dark theme elements
_content/css: improve contrast for playground and dark theme elements

For golang/go#34601.
Fixes golang/go#51584.

Change-Id: I0a1715eacb5e3cef51aff7285dbdf9a0a5a6ea3c
Reviewed-on: https://go-review.googlesource.com/c/website/+/391535
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Jamal Carvalho <19dd88ea049b99d7eb75a23cc7da743ca525bca4@google.com>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website