Update styles.css from FSharp.Data

Here's the current styles:

![image](https://cloud.githubusercontent.com/assets/738761/2790852/673f5b50-cbc0-11e3-9176-8bd3bdd7a5a9.png)

Here's the new styles:

![image](https://cloud.githubusercontent.com/assets/738761/2790859/6fe72fe4-cbc0-11e3-8eb6-ae51f6617bcf.png)

Compare `Type` and `Introduction`. The new one doesn't have aliasing artifacts on Chrome. On other browsers the difference between the two is minimal, as they have much stronger antialiasing than Chrome.