Switch to new header image.
Shrink header slightly on smaller screens.
Fix page background color styling.

It was previously gray so that negative rubber band scrolling in Safari would
show gray, but that's not actually interesting/worth it.
Digest styling improvements.
Make header into a link.
Make style JSON parse errors not be fatal.

Avoids having to restart the server when in development.
Reduce the size of the digest email HTML.

Make monospace be the default font, and only override it in the few places where
we don't want it (as opposed to repeating it for every commit).