color-scheme support for rustup.rs

[Nemo157]

Problem you are trying to solve

Visiting rustup.rs from a dark-theme device can be blinding as it is an almost entirely plain white page.

Solution you'd like

A simple @media (prefers-color-scheme: dark) based dark-mode style. The main rust-lang.org site seems to lack one as well (but is tempered a bit by the big blocks of background color reducing the overall brightness), but https://blog.rust-lang.org/ has a theme that I assume is consistent with the rest of the project.

Notes

No response

[djc]

Sounds good to me -- happy to review a PR for this!

[apiraino]

@Nemo157 thank for mentioning my work on the blog (with the help of other, I'm proud to have contributed a dark theme there 😃 ). Please free to steal anything you may find useful there!

Dark colors on the blog were choosen based on what a browser extension (Dark Reader) suggested me at the time. To my knowledge we don't have a standard color palette for our websites, so you can take the blog as a reference.