@media (max-width: 781px) {
  .wp-block-columns {
    flex-direction: row !important; /* 縦並びを打ち消す */
  }
    .wp-block-columns:not(.is-not-stacked-on-mobile) > .wp-block-column {
    flex-basis: 33.33% !important;
    flex-grow: 1 !important;
  }
}
@media (max-width: 500px) {
  .wp-block-columns {
    flex-direction: column !important; /* 縦並びを打ち消す */
  }
    .wp-block-columns:not(.is-not-stacked-on-mobile) > .wp-block-column {
    flex-basis: 100% !important;
    flex-grow: 1 !important;
  }
}