CSS Variables?

[majapw]

In <v13.0.0 of react-dates, we supported an idea similar to theming in the prebuilt sass stylesheet. Specifically, we had a variables.scss of sass variables with a !default flag and then the consuming stylesheet of classes applied to the various components.

I think that there is an opportunity here to have this CSS interface do a similar thing with the theme variables. They implementation could be CSS variables or something similar, but it would benefit consumers interested in overriding aspects of a project shipping with react-with-styles-interface-css without forcing the consumer to do their own theme and interface registration.