Markup:

<hr class="wp-block-separator">

CSS:

.wp-block-separator {
	border: none;
	border-bottom: 2px solid $dark-gray-100;
	max-width: 100px;
	margin: 1em auto;
}

Many themes have styles for <hr> element so this will have styling issues with many themes. I propose removing styles from the front-end. Yeah I know then it would not match styles in the Gutenberg. That's something we need to solve.