Various CSS tweaks:

* add missing vendor prefixed properties for older browsers support
* use the shorthand properties in a couple of places
* be more explicit with the properties
* add a new variable for white color

Only issue I had was with [this](https://github.com/XhmikosR/david-www/commit/4bbb2414d83e3a49d8ce719fd21d1e7568992d95#diff-a7ed081a2f16b92d910e08ce2ef062ceR19)... Maybe it's fixed in lessjs 1.5.0 though, I'll try it when the grunt plugin is updated.

We should also add a fallback color when rgba is used for older browsers.