Expose the translucent neutral scale as bg-/text-/border-sand-a1
... sand-a12, mapped to the imported --sand-a* variables so they
switch light/dark like the solid steps. Enables overlays, scrims,
and layered tints without hardcoded rgba. Limited to sand; the
other hues keep only their solid steps for now.
Signed-off-by: Émile Ré <emile@probo.com>
48ff1b6ea7