wp-includes/blocks/image/theme.css

:root :where(.wp-block-image figcaption){
  color:#555;
  font-size:13px;
  text-align:center;
}
.is-dark-theme :root :where(.wp-block-image figcaption){
  color:#ffffffa6;
}

.wp-block-image{
  margin:0 0 1em;
}