MAIN.html

<html>
    <head>
        <title>MAIN</title>
        <link rel="stylesheet" href="MAIN.css">

        <style>
           
        </style>

    </head>
    <body class="bg">
        <div class="login">
           <center>

                <img src="account.png" id="user_img" height="130px" width="130px">
                <p id="user" style="color:white;font-size:  20px;font-family:sans-serif;">Athul Nambiar</p>

         

        <a href="DESK.html"><button id="button" onclick="mainFun()">SIGN IN</button></center></a> 
        </div>

        <script>


        //    let btnwal = document.querySelector('#button');
         //   btnwal.addEventListener('click', ()=>{
          //      document.body.style.backgroundImage = "url('images/wall2.jpg')";
           //  }); 
//
             
const btn = document.getElementById('button');
const bgwal = document.getElementsByClassName('bg');

            function mainFun() {
                btn.style.display = 'none';
                btn.style.visibility = 'hidden';
                bgwal.style.backgroundImage = "url(wallpaper.jpg)";
            }

        </script>

    </body>
</html>