Add dark mode support for the website

[zamazan4ik]

Hi!

Some people (me included) prefer to use dark-mode websites for various reasons (I personally use dark mode as a more eyes-friendly mode). I use browser extensions for that like Dark Reader but they don't work wonderfully with the perf.rust-lang.org : color auto-replacement change the website appearance to something unrecognizable enough on graphs.

In this issue, I propose adding a dark mode toggle to the website that will switch between light and dark modes. By default, we can leave the light theme as it is right now.

I understand that the priority for the feature will be too low but maybe someday it will be implemented.