Fix table at end of post.

An earlier version of the markdown parser accepted `code` in table
cells, but now they must be specified using HTML <code></code>.
Put content in a narrower, centered div.
CSS changes:

- General:
  - Set heading font sizes in percentages.
  - Make .code background even lighter.

- Source code:
  - Shrink the font from 80% to 75%.
  - Use a vaguely solarized palette.
  - Make the line numbers less prominent.
Regenerate HTML.