Control backgrounds are alpha stops of one ink; containers paint opaque or nothing

--jolly-control-bg and its hover, focus and active variants are color-mix of a single --jolly-ink at 8%, 12%, 20% and 26%, composited over whatever plane the control lands on. One decision instead of six, and a control stays coherent at any nesting depth without each level picking its own opaque pair. --jolly-invalid-* is the same mechanism with --jolly-ink-danger, starting above the hover stop so an error never reads as hover.

The invariant that makes it work: containers paint opaque or paint nothing, only leaves tint. Two translucent layers over each other drift lighter. Planes (jolly-dock, jolly-rail, jolly-dialog, jolly-floating) paint a surface; in-pane containers (jolly-folder, jolly-tabs) paint nothing; jolly-pane is the one hybrid, nulled out through ::slotted when a plane contains it.

Consequences

Separation between containers comes from leaf tint plus --jolly-row-gap, not from rules. Only dialog headers and footers keep a divider line, and that divider is deliberately faint: at 3:1 it reads as a hard rule between every row of a sixty-control pane.