body{
    margin:0;
    background:#111;
    font-family: Arial;
    display:flex;
    justify-content:center;
    align-items:center;
    height:100vh;
}

.box{
    background:#222;
    padding:20px;
    border-radius:15px;
    width:280px;
}

#screen{
    width:100%;
    height:55px;
    font-size:22px;
    margin-bottom:12px;
    text-align:right;
    padding:8px;
    border:none;
    border-radius:8px;
    background:black;
    color:lime;
}

.keys{
    display:grid;
    grid-template-columns:repeat(4,1fr);
    gap:8px;
}

button{
    height:55px;
    border:none;
    background:#333;
    color:white;
    font-size:18px;
    border-radius:8px;
}

button:hover{
    background:#444;
}

.eq{
    background:green;
}