Themeable checkboxes

The red here is just for the sake of example.
![themed checkboxes 1](https://f.cloud.github.com/assets/3158606/1782230/acddece8-68a7-11e3-873b-1cf512caddf0.png)

They are slightly fuzzier round the edges than the usual ones are. I think I know why, but I'm not sure how to correct it.