how to override parent css in child code example

Example 1: change parent div css on over of child

#main-menu > li:hover > a
{
  background-color: #F00;
}

#main-menu >  li > .submenu > li:hover
{
  background-color:#00F;
}

Example 2: change parent div css on over of child

<ul id="main-menu">
  <li>
    <a href="#">
      <i class="fa fa-building-o" aria-hidden="true"></i>
      Private Limited
      <i class="fa fa-caret-down"></i>
    </a>
    <ul class="submenu">
      <li><a href="#0">Company</a>
      </li>
      <li><a href="#0">Contact</a>
      </li>
      <li><a href="#0">Industry</a>
      </li>
    </ul>
  </li>
</ul>