.wp-block-columns {
    position: relative;

}

.wp-block-columns::before,
.wp-block-columns::after {
    content: '';
    position: absolute;
    width: 100%;
    height: 100%;
}

.wp-block-columns::before {
    left: -100%;

}

.wp-block-columns::after {
    right: -100%;
}