[Blank Canvas Blocks] Evaluate 'base' CSS variable generation

[pbking]

Blank Canvas Blocks has expectations of the availability of particular CSS variables generated from the "custom" block of theme.json. If those variables aren't defined and available then the page won't be correct.

Until we have "theme.json castcading" any theme that leverages Blank Canvas Blocks as a parent will either:

• BE REQUIRED to express all variables in the parent theme.json (including any new ones)
• need to have all default css variables defined by default which is also loaded with the polyfill CSS.

This task is to explore the second option. Hopefully as something that could happen at Blank Canvas Blocks' "build" cycle.

[pbking]

The BESTEST solution seems to be represented here: WordPress/gutenberg#30147

I'm going to close this in lieu of that proven solution.