Step 2 of #2972. `:root` gains `--focus-ring-inner-width` and
`--focus-ring-box-shadow`; `focus-ring()` and the `$*-focus-box-shadow`
aliases resolve to the token, so the ring retunes at runtime and per
theme. The `.focus-ring` helper stops referencing the never-emitted
`--focus-ring-x/-y/-blur`.
CSS diff limited to focus declarations and the `:root` tokens; rendered
values unchanged. test:scss, lint and html-diff green.
Bartek
d52a3e232e