<nav class="main-navigation">

    <ul class="navigation">
      <li class="navigation-item">
        <a class="navigation-link" href="catalog.html">Каталог</a>
        <ul class="submenu">
          <li class="submenu-item-special">
            <a class="submenu-link" href="#">Новинки</a>
          </li>
          <li class="submenu-item">
            <a class="submenu-link" href="catalog.html">Сливочное</a>
          </li>
          <li class="submenu-item">
            <a class="submenu-link" href="#">Щербеты</a>
          </li>
          <li class="submenu-item">
            <a class="submenu-link" href="#">Фруктовый лед</a>
          </li>
          <li class="submenu-item">
            <a class="submenu-link" href="#">Мелорин</a>
          </li>
        </ul>
      </li>
      <li class="navigation-item">
        <a class="navigation-link" href="#">Доставка и оплата</a>
      </li>
      <li class="navigation-item">
        <a class="navigation-link" href="#">О Компании</a>
      </li>
    </ul>
  </nav>



   .submenu {
  display: flex;
  flex-direction: column;
  list-style: none;
  min-width: 143px;
  min-height: 172px;
  justify-content: space-around;
  margin: 0;
  padding: 0;
  background-color: #f8f7f4;
  border-radius: 5px;
  box-shadow: 0px 20px 20px 0px rgba(0, 0, 0, 0.4);
}

.flex-container {
display: flex;
flex-direction: column;
flex-wrap: nowrap;
justify-content: flex-start;
align-content: stretch;
align-items: flex-start;
}