Add CSS custom properties client-side support for older browsers

[marchant]

Integrating something like css-vars-ponyfill @ https://jhildenbiddle.github.io/css-vars-ponyfill/#/?id=variables at the component level.

[marchant]

However, css-vars-ponyfill Custom property declaration support is limited to :root and :host rulesets, so we might need to take a stronger ownership, trying to reuse as much as possible.