Publish button not enabled on theme switch

I tried previewing a different theme and the Publish button was not enabled so I couldn't activate the new theme unless I first dirtied a setting:

![image](https://cloud.githubusercontent.com/assets/134745/23820887/d5010090-05d7-11e7-8c58-307f556b5e20.png)
