Fix diff highlighting

The solarized syntax highlighting we implemented did not include anything for diffs, so Redmine defaults are applying, and it's not pretty.

![screen shot 2014-01-24 at 2 49 45 pm](https://f.cloud.github.com/assets/1437322/1999276/7aad86b4-8541-11e3-83a9-74fffbc915d7.png)

The default scheme is light, so need to tweak this for dark colors.

Selectors:
```
// delete/red
.diff_out, .diff_out span

//add/green
.diff_in, .diff_in span
```