WordPress/wp-includes/blocks/separator/theme.css

.wp-block-separator.has-css-opacity{
  opacity:.4;
}

.wp-block-separator{
  border:none;
  border-bottom:2px solid;
  margin-left:auto;
  margin-right:auto;
}
.wp-block-separator.has-alpha-channel-opacity{
  opacity:1;
}
.wp-block-separator:not(.is-style-wide):not(.is-style-dots){
  width:100px;
}
.wp-block-separator.has-background:not(.is-style-dots){
  border-bottom:none;
  height:1px;
}
.wp-block-separator.has-background:not(.is-style-wide):not(.is-style-dots){
  height:2px;
}