Can't add background of a div in css

Question

I want to add background-color: #f8f8f8 class in step class. But when i added it then black horizontal line is not displaying after 1, 2. Where is the actual problem?

.main-progress {
    text-align: center;
    position: relative;
    margin-top: 30px;
}


.step{
 
}

/*progressbar*/
#progressbar {
    margin-bottom: 30px;
    overflow: hidden;
    /*CSS counters to number the steps*/
    counter-reset: step;
}

#progressbar li {
    list-style-type: none;
    color: #12bd2a;
    font-size: 14px;
    width: 33.33%;
    float: left;
    position: relative;
    font-weight: 600;
}

#progressbar .not-active:before {
    color: #ffffff;
    background: #4650ec;
}

#progressbar li:last-child{
  color: #4650ec;
}

#progressbar li:before {
    content: counter(step);
    counter-increment: step;
    width: 36px;
    height: 36px;
    line-height: 36px;
    display: block;
    font-size: 16px;
    color: #ffffff;
     background-color: #12bd2a;
    border-radius: 25px;
    margin: 0 auto 10px auto;
}

/*progressbar connectors*/
#progressbar li:after {
    content: '';
    width: 100%;
    height: 5px;
    background: black;
    position: absolute;
    left: -50%;
    top: 16px;
    z-index: -1; /*put it behind the numbers*/
}

#progressbar li:first-child:after {
    /*connector not needed before the first step*/
    content: none;
}

/*marking active/completed steps green*/
/*The number of the step and the connector before it = green*/
#progressbar li.active:before, #progressbar li.active:after {
    /*background-color: #12bd2a;*/
    color: white;
}

  <section class="step">
    <div class="container">
      <div class="row">
        <div class="col-12">
          <div class="main-progress">
            <ul id="progressbar">
              <li class="active">Upload Photos</li>
              <li class="active">Model Settings</li>
              <li class="not-active">Get Photos</li>
            </ul>
          </div>
        </div>
      </div>
    </div>
  </section>

Answer 1

try this code

css

    .main-progress {
    text-align: center;
    position: relative;
    margin-top: 30px;
    z-index:9;
}


.step{
 background-color: #f8f8f8;
}

/*progressbar*/
#progressbar {
    margin-bottom: 30px;
    overflow: hidden;
    /*CSS counters to number the steps*/
    counter-reset: step;
}

#progressbar li {
    list-style-type: none;
    color: #12bd2a;
    font-size: 14px;
    width: 33.33%;
    float: left;
    position: relative;
    font-weight: 600;
}

#progressbar .not-active:before {
    color: #ffffff;
    background: #4650ec;
}

#progressbar li:last-child{
  color: #4650ec;
}

#progressbar li:before {
    content: counter(step);
    counter-increment: step;
    width: 36px;
    height: 36px;
    line-height: 36px;
    display: block;
    font-size: 16px;
    color: #ffffff;
     background-color: #12bd2a;
    border-radius: 25px;
    margin: 0 auto 10px auto;
}

/*progressbar connectors*/
#progressbar li.active:after {
    content: '';
    width: 100%;
    height: 5px;
    background:black;
    position: absolute;
    left: -50%;
    top: 16px;
    z-index: -1; /*put it behind the numbers*/
}
#progressbar li.not-active:after {
    content: '';
    width: 100%;
    height: 5px;
    background: black;
    position: absolute;
    left: -50%;
    top: 16px;
    z-index: -1; /*put it behind the numbers*/
}

#progressbar li:first-child:after {
    /*connector not needed before the first step*/
    content: none;
}

/*marking active/completed steps green*/
/*The number of the step and the connector before it = green*/
#progressbar li.active:before, #progressbar li.active:after {
    /*background-color: #12bd2a;*/
    color: white;
}

Answer 2

You see the only problem to your code was the z-index, you put the line before the .step in the z-index. Might be good if you put some z-index to all affected elements.

.main-progress {
  text-align: center;
  position: relative;
  margin-top: 30px;
}

.step {
  background: grey;
  z-index: -1;
}


/*progressbar*/

#progressbar {
  margin-bottom: 30px;
  overflow: hidden;
  /*CSS counters to number the steps*/
  counter-reset: step;
}

#progressbar li {
  list-style-type: none;
  color: #12bd2a;
  font-size: 14px;
  width: 33.33%;
  float: left;
  position: relative;
  font-weight: 600;
}

#progressbar .not-active:before {
  color: #ffffff;
  background: #4650ec;
}

#progressbar li:last-child {
  color: #4650ec;
}

#progressbar li:before {
  content: counter(step);
  counter-increment: step;
  width: 36px;
  height: 36px;
  line-height: 36px;
  display: block;
  font-size: 16px;
  color: #ffffff;
  background-color: #12bd2a;
  border-radius: 25px;
  margin: 0 auto 10px auto;
}


/*progressbar connectors*/

#progressbar li:after {
  content: '';
  width: 100%;
  height: 5px;
  background: black;
  position: absolute;
  left: -50%;
  top: 16px;
  z-index: 1;
  /*put it behind the numbers*/
}

#progressbar li:first-child:after {
  /*connector not needed before the first step*/
  content: none;
}


/*marking active/completed steps green*/


/*The number of the step and the connector before it = green*/

#progressbar li.active:before,
#progressbar li.active:after {
  /*background-color: #12bd2a;*/
  color: white;
}

<section class="step">
  <div class="container">
    <div class="row">
      <div class="col-12">
        <div class="main-progress">
          <ul id="progressbar">
            <li class="active">Upload Photos</li>
            <li class="active">Model Settings</li>
            <li class="not-active">Get Photos</li>
          </ul>
        </div>
      </div>
    </div>
  </div>
</section>

Answer 3

#progressbar:before,#progressbar:after{content:"";display:block;clear:both;}

Add this line and then you will be able to add. It will work for sure.