spread.css

/*  body  */

*{
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}
    body{
    margin: 0;
    display: flex;
    justify-content: center;
    height: 100vh;
    background-color: antiquewhite;
}


/*  calculator */
#calculator{
    font-family: Arial, sans-serif;
    background-color: rgb(7, 7, 14);
    border-radius: 15px;
    max-width: 500px;
    overflow:hidden;
}
/*  display */
#display{
    width: 100%;
    padding:20px;
    font-size: 4rem;
    text-align: left;
    border:none;
    background-color: hsl(0,0%,20%);
    color: white;
}
/*  keys */
#keys{
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap:10px;
    padding:20px;
}

/*  button */
button{
    width:100px;
    height:88px;
    border-radius: 50px;
    border:none;
    background-color: gray;
    color:white;
    font-size: 3rem;
    font-weight: bold;
    cursor:pointer;
}
button:hover{
    background-color: hsl(0,0%,40%);
}
button:active{
    background-color: hsl(0,0%,50%);
}
.operator-btn{
    background-color:hsl(35, 100%, 55%);
}
.operator-btn:hover{
    background-color: hsl(35, 100%, 65%);
}
.operator-btn:active{
    background-color:  hsl(35, 100%, 75%);
}