fix(carbon): address review feedback from WalkQuackBack

- Revert @Version to the template default (2000.01.01); CI bumps it.
- Drop the unused #lib.css-variables() call since nothing here reads
  --ctp-* custom properties.
- Replace the `* { scrollbar-color: ... }` wildcard with a plain
  root-level declaration.
- Remove all !important flags, instead tripling the :root selector
  at the mixin call site so every rule inherits enough specificity
  to win without it.

Author: sixuan911