* {
    box-sizing: border-box;
}

body {
    font-family: 'Roboto', sans-serif;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background-color: #f4f4f9;
    margin: 0;
    padding: 0;
}

.calculator {
    width: 320px;
    background-color: #000;
    padding: 1.5rem;
    border-radius: 1rem;
    box-shadow: 0 8px 16px rgba(0, 0, 0, .1);
}

.calculator-screen {
    display: flex;
    flex-direction: column;
    height: 3.5rem; 
    display: flex;
    align-items: center;
    justify-content: center;
    margin-bottom: 2rem;
}

.calculator-display {
    width: 100%;
    height: 100%;
    font-size: 2.5rem;
    color: #f4f4f9;
    border-radius: .5rem;
    background-color: transparent;
    text-align: right;
    border: none;
}

.display-result{
    font-size: 1.5rem;
    color: rgb(106, 106, 106);
}

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

button {
    height: 4rem;
    font-size: 1.5rem;
    border-radius: 1rem;
    border: none;
    background-color: #2a2a2c;
    color: #fff;
    transition: background-color .3s ease;
}

button:hover {
    background-color: #4a4a4e;
}

.operator { 
    background-color: #f2a33c;
}

.equal-sign { 
    background-color: #1e90ff;
    color: #f4f4f9;
}

.zero {
    grid-column: span 2;
}