Не получается задавать стиль по нажатию кнопки (js)

let currentTab = 0
showTab(currentTab);

const next = document.querySelector('.next'),
      prev = document.querySelector('.prev');

function showTab(n) {
  let x = document.querySelectorAll('.tab');
  x[n].style.display = "block";
  if (n == 0) {
    document.querySelector('.prev').style.display = "none";
  }
}

next.addEventListener('click', () => {
  let x = document.querySelectorAll('.tab');
  if (currentTab <= 2) {
    x[currentTab].style.display = "none";
    currentTab = ++currentTab;
    x[currentTab].style.display = "block";
  }
step();
});


function step(n) {
  var i, x = document.getElementsByClassName("step");
  for (i = 0; i < x.length; i++) {
    x[i].style.backgroundColor = '#bbbbbb';
  }
  x[n].style.backgroundColor = 'red';
}

* {
    box-sizing: border-box;
    padding: 0;
    margin: 0;
}

body {
    font-family: sans-serif;
    background-color: #F9F9F9;
}

/*================================================================================================================
==================================================================================================================*/

#regForm {
  background-color: #ffffff;
  margin: 100px auto;
  font-family: Raleway;
  padding: 40px;
  width: 70%;
  min-width: 300px;
}

h1 {
  text-align: center;  
}

input {
  padding: 10px;
  width: 100%;
  font-size: 17px;
  font-family: Raleway;
  border: 1px solid #aaaaaa;
}

/* Mark input boxes that gets an error on validation: */
input.invalid {
  background-color: #ffdddd;
}

/* Hide all steps by default: */
.tab {
  display: none;
}

button {
  background-color: #4CAF50;
  color: #ffffff;
  border: none;
  padding: 10px 20px;
  font-size: 17px;
  font-family: Raleway;
  cursor: pointer;
}

button:hover {
  opacity: 0.8;
}

#prevBtn {
  background-color: #bbbbbb;
}

/* Make circles that indicate the steps of the form: */
.step {
  height: 15px;
  width: 15px;
  margin: 0 2px;
  background-color: #bbbbbb;
  border: none;  
  border-radius: 50%;
  display: inline-block;
  opacity: 0.5;
}

.step.active {
  opacity: 1;
}

/* Mark the steps that are finished and valid: */
.step.finish {
  background-color: #4CAF50;
}

<form id="regForm" action="/action_page.php">
  <h1>Register:</h1>
  <!-- One "tab" for each step in the form: -->
  <div class="tab">Name:
    <p><input placeholder="First name..." name="fname"></p>
    <p><input placeholder="Last name..." name="lname"></p>
  </div>
  <div class="tab">Contact Info:
    <p><input placeholder="E-mail..." name="email"></p>
    <p><input placeholder="Phone..." name="phone"></p>
  </div>
  <div class="tab">Birthday:
    <p><input placeholder="dd" name="dd"></p>
    <p><input placeholder="mm" name="nn"></p>
    <p><input placeholder="yyyy" name="yyyy"></p>
  </div>
  <div class="tab">Login Info:
    <p><input placeholder="Username..." name="uname"></p>
    <p><input placeholder="Password..." name="pword" type="password"></p>
  </div>
  <div style="overflow:auto;">
    <div style="float:right;">
      <button type="button" class="prev">Previous</button>
      <button type="button" class="next">Next</button>
    </div>
  </div>
  <!-- Circles which indicates the steps of the form: -->
  <div style="text-align:center;margin-top:40px;">
    <span class="step"></span>
    <span class="step"></span>
    <span class="step"></span>
    <span class="step"></span>
  </div>
</form>