I added a event listener to a button, which would change the css of the #popUpForm, but it only works, if I add inline css to the #popUpForm. How could I do it, If i want to set the display of the element in the css?

```js
const addBtn = document.querySelector('#addItem')

addBtn.addEventListener('click', function() {
  document.querySelector('#popUpForm').style.display = 'block'
})
```

```css
#popUpForm {
  width: 350px;
  height: 500px;
  display: none; /* change this */
}
```

```html
<input id="addItem" type="button" value="+">

<div id="popUpForm">
  Some stuff
</div>
```