Collapse border between (+) and (-), alpha bg

Right now both `(+)` and `(-)` have a 1px border, so where they bump together it's 2px.

![before](https://cloud.githubusercontent.com/assets/1378/3361792/7a034d8e-fb02-11e3-89d4-9a0cbea5488a.png)

And let the image come through the controls a bit, it's more interesting than `#222`

![after](https://cloud.githubusercontent.com/assets/1378/3361785/5e78c4a4-fb02-11e3-90fd-13ecba821dcd.png)

@tomborger what do you think?