Added structure for a '.cl-dark' theme

- Can be applied globally to the site (e.g. <html class="cl-dark">)
- Using this to override body background-color and default text color.
![free_online_creative_class_-_live_video_broadcast___creativelive_-_learn__be_inspired_](https://cloud.githubusercontent.com/assets/700114/3345104/c2248bd8-f8b3-11e3-9c76-5ea7dd21d256.png)

I think it's ok in typography- we've got other font colors in there.