body {
    font-family: Arial, sans-serif;
    background-color: #f3f3f3;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
}

/* main calc box*/
.calculator {
    background-color: #222;
    border-radius: 20px;
    overflow: hidden;
    box-shadow: 0 8px 20px rgba(0, 0, 0, 0.3);
}

/*display screen */
.display {
    background-color: #000;
    color: #0f0;
    font-size: 2rem;
    padding: 20px;
    text-align: right;
    height: 60px;
    overflow-x: auto;
}
/*button layout*/
.buttons {
    display: grid;
    grid-template-columns: repeat(4, 80px);
    grid-gap: 1px;
    background-color: #444;
}

.btn {
    height: 80px;
    font-size: 1.5rem;
    background-color: #fff;
    border: none;
    cursor: pointer;
    transition: background-color 0.2s ease;
}

/* + - ÷ ×*/
.operator {
    background-color: green;
    color: white;
}

/* =*/
.equal {
    background-color: #ddd;
    color: white;
}

/*hover and avtive effects*/
.btn:hover {
    background-color: #ddd;
}
.btn:active {
    opacity: 0.7;
}