IconThemeData merge(IconThemeData other)

Returns a new icon theme that matches this icon theme but with some values replaced by the non-null parameters of the given icon theme. If the given icon theme is null, simply returns this icon theme.

Source 

IconThemeData merge(IconThemeData other) {
  if (other == null)
    return this;
  return copyWith(
    color: other.color,
    opacity: other.opacity,
    size: other.size
  );
}