📄 theme.css

.wp-block-video figcaption{
  color:#555;
  font-size:13px;
  text-align:center;
}
.is-dark-theme .wp-block-video figcaption{
  color:hsla(0,0%,100%,.65);
}

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