function wp_admin_add_separators(){
	?>
	<script type="text/javascript">
		var addSeparators = function(){
			jQuery('#adminmenu > li').each(function(){
				var $this = jQuery(this);
				var $next = $this.next();
				if($next && !$this.hasClass('wp-menu-separator') && !$next.hasClass('wp-menu-separator')){
					$next.before('<li class="wp-not-current-submenu wp-menu-separator" aria-hidden="true"><div class="separator"></div></li>');
				}
			});
		};

		var checkForSeparators = setInterval(function(){
			if(jQuery('#adminmenu').size()){
				addSeparators();
				clearInterval(checkForSeparators);
			}
		}, 20);
	</script>
	<?php
}

add_action('admin_head', 'wp_admin_add_separators');

Please add this code to functions.php
This will add admin menu separators between all admin menu items.

Add Admin Menu Separators

plugin

PHP