Adds a span around post counts in category archives widget and general archives widgets. This allows for easy styling of the count—for instance, using leading dots. (The SCSS file adds leading dots.)

add-span-to-count.php

/**
 * Filter the categories archive widget to add a span around post count
 */
function smittenkitchen_cat_count_span( $links ) {
	$links = str_replace( '</a> (', '</a><span class="post-count">(', $links );
	$links = str_replace( ')', ')</span>', $links );
	return $links;
}
add_filter( 'wp_list_categories', 'smittenkitchen_cat_count_span' );

/**
 * Filter the archives widget to add a span around post count
 */
function smittenkitchen_archive_count_span( $links ) {
	$links = str_replace( '</a>&nbsp;(', '</a><span class="post-count">(', $links );
	$links = str_replace( ')', ')</span>', $links );
	return $links;
}
add_filter( 'get_archives_link', 'smittenkitchen_archive_count_span' );

archive-widgets.scss

/* Archive widgets */
.widget_archive,
.widget_categories {
	ul li {
		align-items: baseline;
		display: flex;
		flex-wrap: wrap;

		a {
			border: none;
			order: 1;

			&:hover {
				background-color: transparent;
			}
		}

		&::before {
			border-bottom: 1px dotted $color__light-grey;
			content: "";
			flex-grow: 1;
			order: 2;
			margin: 0.25em 3px;
		} // :before

		.post-count {
			order: 3;
			padding-top: 0.5em;
		}

		ul {
			order: 4;
			flex-grow: 0;
			flex-basis: 100%;
			margin-left: 1em;
		}
	} // ul li
} // .widget_archive, .widget_categories

Nice. Thank you!