How to add class via Javascript instead of work with css inside js code?

Question

I'm building a sidenav menu. I want to work on the style directly in the css file instead of the Javascript code as below.

For example: dropContent.style.display === "block" and "none", I wish there was a class to modify and not add css values inside the js code. Is this possible ?

References: https://www.w3schools.com/howto/tryit.asp?filename=tryhow_js_sidenav_dropdown

var dropdown = document.getElementsByClassName("dropdown-btn");
var i;

  for (i = 0; i < dropdown.length; i++) {
  dropdown[i].addEventListener("click", function() {
  this.classList.toggle("activate");
  
  var dropContent = this.nextElementSibling;
  if (dropContent.style.display === "block") {
      dropContent.style.display = "none";
    } else {
      dropContent.style.display = "block";
    }
  });
}