Whiteboard night mode

Modification of #481 to include night mode support for whiteboard icons, but with different icons. It would be nice to have a better clear icon which won't be confused with delete note, but I think this will do for now.

![normal](https://cloud.githubusercontent.com/assets/2818274/4415430/12eba9e8-452b-11e4-9585-3ffe0cfd2ce3.png)

![night](https://cloud.githubusercontent.com/assets/2818274/4415432/17cfa28e-452b-11e4-9d4f-1baefb2afddb.png)