Stubbing something out to close out some issues and setup some next steps. While we won't be using this just yet, we can in the future. I'm not 100% sure on custom color schemes mind you, so that could be all for naught. Choose the Custom > Sass page for this, and in the future, other mixins that are not attached to components and utilities. So for now, it's a draft.
Fixes #29551 (closed). Successor to #29552.
X-ref #32747 for documenting more variables, mixins, etc.