chore: light background for code snippets

[dummdidumm]

While working on tweaking dark mode I noticed that I find the content much easier to scan. Turns out it's because we don't give code blocks any background whatsoever, which in my view is a mistake. It makes the code too easy to mix up with the surrounding text when scrolling and searching for something.

This gives the code blocks a very very light grey background, which in my opinion achieves a sweet spot between making it easier to separate code from text while not getting in the way. (The fact that there was a commented-out --sk-back-3 makes me believe this was actually something on the todo list or something that was contemplated to do)

[Rich-Harris]

The background was removed for the sake of the syntax highlighting, which suffers a bit with a background (except on dark mode, where the colours pop). I think the box works but needs to be a smidge lighter, and the bar for the filename feels too prominent.

Since those colours don't exist in our palette at the moment, I suspect we need to revisit our tokens in light of this and #699 — we have a bunch of different --sk-text-n and --sk-back-n tokens but no cohesive theory about what we're supposed to use when, and the colours themselves feel a bit random (e.g. tiny gap between 1, 2, 3 then a big jump to 4). Some uses (borders, bullets) are neither text nor back but a third thing.

[Rich-Harris]

merged #765, closing