Light gray bg for white code scheme if used in comments and wiki

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'master' into improve-white-code-highlight
Merge branch 'improve-white-code-highlight' into 'master'

Improve white code highlight in comments and wiki

![Screenshot 2014-11-05 16.36.55](https://dev.gitlab.org/uploads/dzaporozhets/gitlabhq/5723848593/Screenshot_2014-11-05_16.36.55.png)

See merge request !1241