Make the diff viewer prettier and cleaner.

This polishes up the look of the diff viewer a bit. The highlights
include:

* Removed lots of unnecessary borders, that separated content, instead
  relying on the backgrounds and whitespace for the separation.

* Softened the colors, making the reds and greens and browns less harsh.

* The borders around the chunks are now based on the color of the chunk,
  instead of a generic grey.

* Removed a lot of the borders on the file header. The header is now one
  box instead of three or four.

* The moved line indicators are simpler and less bulky. The font size
  has been reduced, the complex background is gone, they have more
  consistent padding, and they are now flush up against each other.

* The table borders are rounded, making them more consistent with the
  other boxy bits of the UI.