Make the homepage bg a little darker

... so that it is not identical with the bg of sidebar

The new version looks like this:

![new_bg](https://f.cloud.github.com/assets/2093792/1009699/fcb57470-0b3c-11e3-8a4c-af4d1a88e3fb.png)

(Instead of this:
![resize_bug](https://f.cloud.github.com/assets/2093792/1009701/063aa272-0b3d-11e3-9985-feac46100215.png)
)

This fixes #664.