Better custom props / css vars

We do support custom props right now, but they have some inherent problems:

1. Declaring `--something` requires quotes in object syntax
1. Using them requires string-based syntax `'var(--something)'`
1. CSS implements them again in the global namespace with cascade and inheritance

**We can keep the current syntax and additionally provide a better one.**

Let's have this conversation started.

The issue concerns JSS custom properties and CSS variables but names no files, tests, entry points, or concrete acceptance criteria. Start by reviewing the existing custom-property support and the four-comment discussion; the work is not defined enough to identify what “done” looks like.

Written by the indexing model from the issue text.