Adjust border-radius of zoom buttons.

The `border-radius` of the actual zoom buttons currently equals the radius of the container they are in. 

I suggest to have a smaller inner radius that IMO looks more visually appealing.

Compare in the following image: The zoom-in button (on top) is adjusted already and the zoom-out (on the bottom) button shows the current border-radius of `4px`.

![border-radius](https://f.cloud.github.com/assets/227934/89247/2ef8b0f2-652d-11e2-9b16-7084dbd7c36e.png)