WIP, but trying to capture some of the work @MartijnCuppens and @ffoodd are doing across multiple PRs. Fellas, please weigh in here on anything else we should be striving to mention. Some thoughts from me thus far mean we could include guidelines for how we decide what gets CSS vars, how to customize and utilize them, how we generate them with Sass, etc.
X-ref: #26596 (closed)