Customize CSS for literal text blocks to have a gray background and border

[carols10cents]

So the bash syntax highlighting does colorize some arbitrary things in cargo output that is kind of confusing, but I still want cargo output blocks to have a gray background and border to set them apart from the prose. I think we can tweak the CSS that the mdbook output uses in order to do this, and then we can change all the bash code blocks to just be plain text.

This doesn't affect the print book.

[steveklabnik]

👍

[carols10cents]

I ended up digging into this some more and reporting it to mdbook upstream, i'm hoping to have a patch in there for a workaround soon.