Coloured block level share icons in galleries

Coloured in those controversial monochrome share icons on gallery pages:

After:
![screen shot 2014-11-24 at 17 08 44](https://cloud.githubusercontent.com/assets/2236852/5169160/93136232-73fc-11e4-84ed-99db70534604.png)


could abstract this '10% darken' patterm into a function, e.g..

`colour-hover(social-pinterest) == darken(colour(social-pinterest), 10%)`