wp-includes/blocks/embed/theme.css

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

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