Dark Core UI

https://trello.com/c/GjJNtQSX/1381-more-thorough-dark-mode

![screen shot 2014-08-11 at 9 51 56 pm](https://cloud.githubusercontent.com/assets/1495261/3886491/6af3f694-21dc-11e4-931a-fe45f0168e7e.png)

This says to "NEVER use color literals (e.g. #aaa) elsewhere in our LESS", but I see `rgb()`, `rgba()`, and `black` are still used.