* {
    box-sizing: border-box;
}

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

.calculator {
    border: 1px solid #ccc;
    border-radius: 5px;
    padding: 20px;
    width: 400px;
    box-shadow: 2px 2px 10px rgba(0, 0, 0, 0.1);
}

.calculator-screen {
    width: 100%;
    height: 80px;
    border: none;
    background-color: #252525;
    color: #fff;
    text-align: right;
    padding: 0 10px;
    font-size: 2.5rem;
    border-radius: 5px;
    margin-bottom: 10px;
}

button {
    height: 60px;
    font-size: 1.5rem;
    margin: 5px;
    flex: 1 1 20%;
    border-radius: 5px;
    border: none;
    background-color: #f1f3f4;
}

button.operator {
    background-color: #f9a825;
    color: #fff;
}

button.equal-sign {
    background-color: #009688;
    color: #fff;
}

button.all-clear {
    background-color: #d32f2f;
    color: #fff;
}

.calculator-keys {
    display: flex;
    flex-wrap: wrap;
}