Serialize a derived theme into overlay CSS scoped to [data-theme-overlay="<id>"].
A single block suffices: it overrides primitives that _tokens.scss's semantic and
dark blocks reference, so both modes re-point automatically.
The optional advanced layer (decision: seed-first, with an escape hatch) is applied
last: overrides merge over the derived vars; customCss is appended, wrapped in the
overlay selector so raw rules stay scoped to this theme.
Serialize a derived theme into overlay CSS scoped to
[data-theme-overlay="<id>"]. A single block suffices: it overrides primitives that _tokens.scss's semantic and dark blocks reference, so both modes re-point automatically.The optional advanced layer (decision: seed-first, with an escape hatch) is applied last:
overridesmerge over the derived vars;customCssis appended, wrapped in the overlay selector so raw rules stay scoped to this theme.