Styling <spark-dialog>

@devoncarew @cleercode

This is just for the dialog frame, it excludes the controls in the body.

 
![screen shot 2014-05-30 at 6 04 35 am](https://cloud.githubusercontent.com/assets/5606182/3130656/1e70ee96-e7fb-11e3-8653-3130d0194c22.png)