Colour
A cool neutral ramp, three ink tones and exactly one accent. Light theme only — the structure is theme-ready, so adding dark means adding a second block of tokens and changing nothing else.
Surfaces
Ordered by depth rather than by lightness. Sunken and inset go down into the page; raised comes up off it, using a hairline and a shadow rather than a lighter fill.
Ink
Three tones, deliberately no more. Ratios are measured against --surface-page, and all three pass WCAG AA for body text — so choosing a quieter tone is a hierarchy decision, never an accessibility compromise.
Accent
One hue, and its job is to mark the single most important thing in view. --accent is the only variant safe for text: 5.0:1 against the page, and white on it clears 5.4:1, so it works as ink and as fill.
Rules
Alpha, never solid. A hairline has to sit on every surface in the ramp and hold the same optical weight, which a solid grey cannot do — it will read too heavy on the sunken surface and vanish on the raised one.