如何在javascript中插入css？

This is my script

这是我的剧本

<script type = "text/javascript">
        function pic1()
        {
            document.getElementById("img").src = "../images/images/1.png";
        }
        function pic2()
        {
            document.getElementById("img").src = "../images/images/2.png";
        }
</script>
<script type = "te