Add CSS variables

Firefox 29 added support for that feature.
https://bugzilla.mozilla.org/show_bug.cgi?id=773296
data update
Merge pull request #397 from MayhemYDG/patch-2

Add CSS variables
data update
Clarify that `/data.json` should not be changed
Merge pull request #422 from mathiasbynens/patch-3

Clarify that `/data.json` should not be changed