Go: x/pkgsite: add support for dark mode

[A-UNDERSCORE-D]

What is the URL of the page with the issue?

All Pages (noticed explicitly on my first use, which was os.ProcessState.Success)

What is your user agent?

Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:72.0) Gecko/20100101 Firefox/72.0

What did you expect to see?

Support for my already configured system wide dark mode (via CSS prefers-colour-scheme)

Or at the very least a toggle button somewhere to enable something less eye-searing for those of us working at night or in the dark

What did you see instead?

I didnt. I was blinded.

But in all seriousness, a beautifully designed, if bright white docs page. Which instantly clashed with the rest of my system as it is configured to have a dark GTK theme, et al.

On the old godoc.org, I managed this though a stylus theme, but I'd much rather have first party support on pkg.go.dev.

[julieqiu]

/cc @andybons @dmitshur

[julieqiu]

We're currently discussing this request with our UX team, along with other design feedback we've received for the site. We want to review design feedback we have received as part of a broader redesign, as opposed to make piecemeal decisions. I'll update this issue once we have decided on any changes.

/cc @fflewddur

[Jacalz]

This is very much needed. I keep burning my eyes on the white background. We all know the memes about developers using dark themes ;)