<style>

body {
background-image: linear-gradient(to right, #9a13e9, #dd09dd);  
text-align: center;
}

button {

background-color: rgb(231, 9, 94);
height: 30;
width: 500;
font-size: 23;
}

</style>

    <body>
 <h1>sangin rangin</h1>

 <button type="button" onclick="this.style.background ='yellow';window.open('../nemoneh.html', false)" >کار های  نمونه</button>

</body>