[HOLD] Dark Core UI

To see the Dark UI:

1. use ```brackets_core_ui_variables_dark.less``` in ```brackets_shared.less```.
2. paste variables of ```brackets_core_ui_variables_dark.less``` into the appropriate core extension stylesheets.

![screen shot 2014-07-14 at 3 24 26 pm](https://cloud.githubusercontent.com/assets/1495261/3578174/a592e366-0ba5-11e4-8b40-2f79293e6d2f.png)

@redmunds For review, you can use this URL: https://github.com/adobe/brackets/pull/8362/files?w=0