Darken BG color for unread notifications

The background color for /notifications/ doesn't contrast enough to
clearly distinguish the unread from the read easily.

I'm not particularly tied to this color, it just looked OK to me.

New: ![2013-10-11-140510_46x11_scrot](https://f.cloud.github.com/assets/947843/1318294/d686f280-32b8-11e3-932d-ee404a47a8c0.png)
Old: ![2013-10-11-140519_52x14_scrot](https://f.cloud.github.com/assets/947843/1318295/d69eaf6a-32b8-11e3-95ce-4753f8c59f4b.png)