The CSS export left out a colour calculation that resolves to nothing but still wrote an alias to
it as var(--name); where no block defines that property, whatever reads the alias is invalid.
CSS (and Tailwind) and Penpot now share one rule, Variables::flattener: an alias that leads to a
calculation some mode of its collection leaves out goes out as the value it shows in the mode,
or is left out when it shows nothing.