Untitled

<!DOCTYPE HTML>
<html>
<head>
    <meta charset="utf-8">
    <title>css</title>

</head>
<body>
<div>
    <ul>
        <li>text</li>
        <li>text</li>
        <li id="elem">text</li>
        <li>text</li>
        <li>text</li>
    </ul>
</div>

<script>
    let style = document.querySelector('#elem').style;
    let css = {
        border: '2px solid red'
    }
    Object.assign(style, css)
</script>
</body>
</html>