style(button): fatten up the button text for improved readability

Button text readability is low on Linux:
![image](https://cloud.githubusercontent.com/assets/706039/4846996/95960700-604f-11e4-80ba-96821b019d58.png)
![image](https://cloud.githubusercontent.com/assets/706039/4847001/a0acf7e8-604f-11e4-99f8-b22523962e9d.png)

---

Before (on a mac, which isn't quite as bad as Linux):
![image](https://cloud.githubusercontent.com/assets/706039/4846987/8b551f6a-604f-11e4-8179-a298201c71c7.png)

After: 
![image](https://cloud.githubusercontent.com/assets/706039/4846985/80e8f790-604f-11e4-9b2d-99b165704861.png)


/cc @velveteer