Add support for css variables

[green-coder]

We may add a grammar for setting css variables, for example:

(* Matches "set-var-foobar-#fff" *)
set-css-variable = <'set-var-'> css-variable-name <'-'> encoded-css-value

or to read a css variable:

read-css-variable = <'var-'> css-variable-name

We may also add the read-css-variable in places where the user may want to use them. Not sure if we should add support for it everywhere or just start with places which require a color and experiment on that.