Fix csslint warnings

Also:
* Added modules/syntaxhighlight to csslintignore because
  it is broken right now, so it's hard to fix those warnings
  without being able to verify it.
* Fixed a typo in the grunt-watch config that accessed an
  inexistent property.

Change-Id: Ib81572506786b6a1203c454d1b2b91bb6ae2a3de