Colours conventions

- [new] #545454 is a variable
- [fix] Apply colour hexa conventions
- [fix] Use `$c-neutral2` (`#767676`) instead of `#666666`

![ ](http://25.media.tumblr.com/582f462152ad2f64f86725c3d14f7ed5/tumblr_n29pkkCYDw1s2yegdo1_400.gif)