<div class="message-computer"><p>Hi, how are you?</p></div>

<div class="message-user"><p>I am good, thanks!</p></div>

<script>
function changeClass(){

      var myElements = document.querySelectorAll(".message-computer");
      for (var i = 0; i < myElements.length; i++) {
        myElements[i].style.color = "black";
        myElements[i].style.backgroundColor = "yellow";
      }

    }
  </script>

<div class="message-computer" style="color: black; background-color: yellow;"><p>Hi, how are you?</p></div>