Assign unique IDs to HTML elements via twig file
Copy /core/themes/bartik/templates/classy/navigation/menu.html.twig
and modify a bit:
{#
/**
 * @file
 * Theme override to display a menu.
 *
 * Available variables:
 * - menu_name: The machine name of the menu.
 * - items: A nested list of menu items. Each menu item contains:
 *   - attributes: HTML attributes for the menu item.
 *   - below: The menu item child items.
 *   - title: The menu link title.
 *   - url: The menu link url, instance of \Drupal\Core\Url
 *   - localized_options: Menu link localized options.
 *   - is_expanded: TRUE if the link has visible children within the current
 *     menu tree.
 *   - is_collapsed: TRUE if the link has children within the current menu tree
 *     that are not currently visible.
 *   - in_active_trail: TRUE if the link is in the active trail.
 */
#}
{% import _self as menus %}

{#
  We call a macro which calls itself to render the full tree.
  @see https://twig.symfony.com/doc/1.x/tags/macro.html
#}
{{ menus.menu_links(items, attributes, 0) }}

{% macro menu_links(items, attributes, menu_level) %}
  {% import _self as menus %}
  {% if items %}
    {% if menu_level == 0 %}
      &lt;ul{{ attributes.addClass('menu') }}&gt;
    {% else %}
      &lt;ul class=&quot;menu&quot;&gt;
    {% endif %}
    {% set count = 1 %} {# &lt;------------------ Added #}
    {% for item in items %}
      {%
        set classes = [
          'menu-item-' ~ count, {# &lt;------------------ Modified #}
          item.is_expanded ? 'menu-item--expanded',
          item.is_collapsed ? 'menu-item--collapsed',
          item.in_active_trail ? 'menu-item--active-trail',
        ]
      %}
      &lt;li{{ item.attributes.addClass(classes) }}&gt;
        {{ link(item.title, item.url) }}
        {% if item.below %}
          {{ menus.menu_links(item.below, attributes, menu_level + 1) }}
        {% endif %}
      &lt;/li&gt;
      {% set count = count + 1 %} {# &lt;------------------ Added #}
    {% endfor %}
    &lt;/ul&gt;
  {% endif %}
{% endmacro %}

Result

Then you can do, Ex: #menu-block-id .menu-item-1 { border: solid green; }