Avoid calc(700px + 9em) in media queries

Avoid calc(700px + 9em) in media queries

I got a report that code blocks looked wrong on mobile. I was able to
reproduce in Chrome: they didn't get their margins properly updated by
the media query. Investigation revealed that apparently calc() with
mixed units is broken in media queries on Chrome.

https://bugs.chromium.org/p/chromium/issues/detail?id=843584

What I'm seeing here seems worse than the issue described there, so
there might be other factors going on, but it worked perfectly in
Firefox so I'm not sure what's going on. >_<