Tidy up grade report floating background colours.

Hi Bas,

Makes the floating bit of the M2.8 grader report look consistent with the background colours:

![2014-11-16 15_35_12-grades_ view](https://cloud.githubusercontent.com/assets/1058419/5061975/b2c9fa34-6da6-11e4-9076-2900c97b657a.jpg)

Cheers,

Gareth