The escape hatch out of the math below. Every derived value here (the gridline, the header hover) is an elevation calculation: it assumes "raised" means "lighter", which is the Material idea. A host with a FLAT design language (VS Code, Fluent) has its own named surfaces that do not follow that curve at all, and mapping them onto the five MUI palette slots the adapter reads throws all of them away. So each derivation is a FALLBACK: an explicitly supplied token wins, and only what the host leaves unset is computed.
Explicit token values a HOST supplies, addressed by cell —
(surface × mode), not just token. So a host says "the header paper iseditorGroupHeader.tabsBackground" without any new token name entering the vocabulary; the expressiveness comes from the matrix that already exists (THEMING-DESIGN §8).