*{
    margin:0;
    padding:0;
    box-sizing:border-box;
    font-family:Arial,sans-serif;
}

body{
    display:flex;
    justify-content:center;
    align-items:center;
    height:100vh;
    background:#111827;
}

.calculator{
    background:#1F2937;
    padding:20px;
    border-radius:15px;
    width:320px;
}

#display{
    width:100%;
    height:60px;
    margin-bottom:15px;
    font-size:24px;
    text-align:right;
    padding:10px;
    border:none;
    border-radius:10px;
    background:#fff;
}

.buttons{
    display:grid;
    grid-template-columns:repeat(4,1fr);
    gap:10px;
}

button{
    padding:18px;
    font-size:20px;
    border:none;
    border-radius:10px;
    cursor:pointer;
    background:#38BDF8;
    transition:.3s;
}

button:hover{
    background:#0EA5E9;
    color:#fff;
}

.back-btn{
    position: fixed;
    top: 20px;
    left: 20px;
    padding: 10px 20px;
    background: #2563EB;
    color: white;
    text-decoration: none;
    border-radius: 5px;
    font-weight: bold;
    z-index: 1000;
}

.back-btn:hover{
    background: #1D4ED8;
}