Shadow DOM 201: Update CSS Variables syntax to the latest one.

The syntax for custom properties changed from `var-foo` to `--foo` in the latest
spec. This updates the syntax used in examples. #1133
Remove the mixins for CSS variables.
Replace `font` with `font-family`.

The `font` shorthand requires at least `font-size` and `font-family` to be specified.
Merge pull request #1134 from myakura/shadowdom-201/cssvariables

Shadow DOM 201: Update CSS Variables syntax to the latest one.