themes-wordpress/blockbase/sass/blocks/_separator.scss

.wp-block-separator {
	opacity: var(--wp--custom--separator--opacity); // Mirror controls that Gutenberg theme.css offers: https://github.com/WordPress/gutenberg/blob/trunk/packages/block-library/src/separator/theme.scss - See: https://github.com/WordPress/gutenberg/issues/34637
	&:not(.is-style-wide){
		width: var(--wp--custom--separator--width); // See https://github.com/WordPress/gutenberg/issues/34638
	}
}