// Since the recipe author might not know ahead of time what theme the
// block is in, they should supply a map whose keys are theme names and
// values are region names, so we know where to place this block. If the
// target theme is not in the map, they should supply the name of a
// fallback region. If all that fails, give up with an exception.
assert(is_array($value['region']));
$value['region']=$value['region'][$theme]??$value['default_region']??thrownewConfigActionException("Cannot determine which region to place this block into, because no default region was provided.");