This updates our syntax highlighting to render well in dark mode, when

placed on a dark background.
The dark theme mirrors the light theme generally, but leans more on

purples, reds, cyans, and oranges as the main colors. This looks good on

a dark background, and on the insert/delete/replace lines (which are

defined in this change but not yet used in the diff viewer).
Some old CSS selectors have been removed in the process (from a time

when we rendered some syntax highlighting client-side), and others have

been added (to try to better align Pygments and CodeMirror styling).
There was a greater plan to make diff themes more pluggable. This work

exists in a branch, but is no longer in scope for Review Board 7.

However, some of the naming in this change prepares for that eventual

support.
As of this change, code is almost impossible to read in the diff viewer

on dark mode, due to the bright colors in the diff viewer. The next

change will add the necessary styling to correct this.

Tested in CodeMirror on light and dark modes, with a variety of code

samples that tested every syntax-highlighted token supported in both

Pygments and CodeMirror.
Tested in the diff viewer as well. Right now, it's unusable, but I've

also tested with the in-progress diff viewer changes.