Change blog-feedback bg color

For [GitHub's blog](https://github.com/blog/1884-introducing-split-diffs).

Before:
![screen shot 2014-09-04 at 14 47 16](https://cloud.githubusercontent.com/assets/867840/4146654/b0c961f6-340e-11e4-89d8-06eb42b17873.png)

After:
![screen shot 2014-09-04 at 15 33 44](https://cloud.githubusercontent.com/assets/867840/4146664/d3bf49be-340e-11e4-8e27-8f19a1f366f7.png)