Styling <spark-dialog>

@cleercode: the look
@devoncarew: I'll submit this for a code review after the preceding CLs land and I rebase this.

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/5606182/3130770/f265a3da-e7fc-11e3-8ef1-ba13e763a5d5.png)