[D9] Add a sub theme starter kit to Basis

Description of the need

I just had some very confusing issues with how color module works in a custom theme I was making based upon Basis (https://github.com/backdrop/backdrop-issues/issues/3341). I think that one would have the same confusion in sub theming basis. I've seen comments (https://github.com/backdrop/backdrop-issues/issues/2225#issuecomment-249...) suggesting that we better document how to sub theme with Basis.

Would it make sense to include a sub theme starter kit in Basis with detailed documentation similar to what the Zen theme for Drupal had?

I understand that many people are already sub theming Basis AND this would fit the strategy of making Backdrop CMS friendly for beginning developers.

The Basis theme is probably the most likely piece of code that a new developer would engage with directly in Backdrop CMS Core.

Proposed solution

We could include this starter kit in the /themes directory which currently ships empty. The starter kit could be focused on Basis, but documented in such a way as to help a user sub theme any theme, using this as a template.

Alternatives that have been considered

This could be a contrib theme entirely and we could just add a note pointing at this contrib sub theme in the themes readme file that comes with core?