<button id="colorBtn">呪いをかけろ</button>
<div id="box"></div>
#box {
width: 120px;
height: 120px;
background: #ccc;
transition: background 0.3s;
}
.cursed {
background: red !important;
border-radius: 50%;
}
const btn = document.querySelector('#colorBtn');
const box = document.querySelector('#box');
btn.addEventListener('click', function() {
box.classList.toggle('cursed');
});