Improve button styling

The buttons across mantis seems washed out with misaligned text. This change sharpens the button and makes them standout and easy to find. It affects all buttons in the product so I suggest trying it out on your dev box as part of the review:

Before:
![screen shot 2014-11-08 at 9 23 44 pm](https://cloud.githubusercontent.com/assets/1354889/4966569/1a2db40c-67d2-11e4-9f43-fcfbcf173798.png)

![screen shot 2014-11-08 at 9 25 09 pm](https://cloud.githubusercontent.com/assets/1354889/4966570/23bb5696-67d2-11e4-8abb-f1a7d07c4311.png)

After:
![screen shot 2014-11-08 at 9 24 36 pm](https://cloud.githubusercontent.com/assets/1354889/4966571/2ca59fa0-67d2-11e4-997f-e543b3af9777.png)

![screen shot 2014-11-08 at 9 25 20 pm](https://cloud.githubusercontent.com/assets/1354889/4966572/307b0bec-67d2-11e4-8d1b-d65251d1e42e.png)


Fixes #17820