Filters the result of saving the newsletter email design. The editor sends the whole styles document rather than a patch, so implementations replace rather than merge. Implementations should report on a read-back of the stored design rather than echoing the submitted one: sanitizing drops properties outside the theme.json schema, so a save can succeed into invisibility, and the screen has to be able to tell a person their edit did not survive. Internal, and not settled, for the same reason as the read above: expect the shape of the design document and of what is returned to change while the editor is being built.