_content/css: invert github css for dark theme
_content/css: invert github css for dark theme

Fixes golang/go#56559.

Change-Id: I3f8aa801ad3ef97686903dd1019f5df2f2ed9e3d
Reviewed-on: https://go-review.googlesource.com/c/website/+/450416
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Auto-Submit: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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