Pseudo-element 'after' can not be seen

I have the following structure:

<div style="background:green">
     <ul class="list-unstyled multi-steps">
        <li>Start</li>
        <li>First Step</li>
        <li class="is-active">Middle Stage</li>
        <li>Finish</li>
    </ul>
</div>

CSS:

        .multi-steps > li.is-active:before, .multi-steps > li.is-active ~ li:before {
  content: counter(stepNum);
  font-family: inherit;
  font-weight: 700;
}
.multi-steps > li.is-active:after, .multi-steps > li.is-active ~ li:after {
  background-color: #ededed;
}

.multi-steps {
  display: table;
  table-layout: fixed;
  width: 100%;
}
.multi-steps > li {
  counter-increment: stepNum;
  text-align: center;
  display: table-cell;
  position: relative;
  color: tomato;
}
.multi-steps > li:before {
  content: '\f00c';
  display: block;
  margin: 0 auto 4px;
  background-color: #fff;
  width: 36px;
  height: 36px;
  line-height: 32px;
  text-align: center;
  font-family: 'FontAwesome';
  border-width: 2px;
  border-style: solid;
  border-color: tomato;
  border-radius: 50%;
}
.multi-steps > li:after {
  content: '';
  height: 2px;
  width: 100%;
  background-color: tomato;
  position: absolute;
  top: 16px;
  left: 50%;
  z-index: -1;
}
.multi-steps > li:last-child:after {
  display: none;
}
.multi-steps > li.is-active:before {
  background-color: #fff;
  border-color: tomato;
}
.multi-steps > li.is-active ~ li {
  color: #808080;
}
.multi-steps > li.is-active ~ li:before {
  background-color: #ededed;
  border-color: #ededed;
}

The content of 'after' is not displayed, but by removing the bakground from the container, if seen.