<button id="toggle">Toggle</button>
<div class="submenu">Submenu</div>
html {
  background: #fbdf00;
  font-size: 20px;
}

.active {
  background-color: white;
}
let toggle = document.querySelector("#toggle");
let submenu = document.querySelector(".submenu");

toggle.addEventListener('click', function() {
   submenu.classList.toggle('active'); 
});

External CSS

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External JavaScript

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