Fix CSS for level 2 navigation

The sidebar navigation CSS is badly broken when documents are used in the `toctree` (as opposed to just section headers). This is due to both a typo in `_theme_layout.sass` as well as a single missing CSS rule.

BEFORE
----------

![screenshot-1](https://f.cloud.github.com/assets/369261/1848783/36b2d15e-769a-11e3-9d2f-7cd5044e7ac7.png)
* * *
![screenshot-4](https://f.cloud.github.com/assets/369261/1848792/552fdf00-769a-11e3-8219-9af20f1c53e9.png)
* * *
![screenshot-5](https://f.cloud.github.com/assets/369261/1848793/571a9724-769a-11e3-8273-371c99d651f3.png)

AFTER
--------

![screenshot-1](https://f.cloud.github.com/assets/369261/1848783/36b2d15e-769a-11e3-9d2f-7cd5044e7ac7.png)
* * *
![screenshot](https://f.cloud.github.com/assets/369261/1848796/8736b4f6-769a-11e3-85f6-785eb82cb174.png)
* * *
![screenshot-3](https://f.cloud.github.com/assets/369261/1848798/8e2a9b06-769a-11e3-9231-5e5f2fcce60f.png)