[Activity] Improved styling

The activities app looks much better now.

Next, I'll look into making the message body look better.

@jancborchardt please review the design, here is a screenshot:
![activitiesstyling](https://f.cloud.github.com/assets/277525/1246672/73666c86-2ab0-11e3-8132-bd126ffb621d.png)