Make negative areas of selection triangle transparent (& other fixes)

Doubled checked again after a PR was merged and realized I missed two spots while I was reviewing. This PR will fix the problem in the screenshots below:

Update: There are also fixes for #8491 and #8494. 

![screen shot 2014-07-21 at 12 57 06 pm](https://cloud.githubusercontent.com/assets/1495261/3649364/bf989eba-1111-11e4-86a1-99d1342784c4.png)

![screen shot 2014-07-21 at 12 53 38 pm](https://cloud.githubusercontent.com/assets/1495261/3649367/c222d3d0-1111-11e4-9235-0516ed5c8259.png)

Am I correct in thinking that this should actually be applied in the core stylesheet with the (not yet there) `dark` class?