body {
    margin: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background-color: hsl(0, 0%, 95%);
}

#calculator {
    font-family: Arial, sans-serif;
    background-color: hsl(0, 0%, 15%);
    border-radius: 15px;
    width: 400px; 
    overflow: hidden;
}

#display {
    width: 100%;    
    padding: 20px;
    font-size: 4rem;
    text-align: left; 
    border: none;
    background-color: hsl(0, 0%, 20%);
    color: white;
    box-sizing: border-box;
    outline: none;
}

#keys {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
    padding: 20px;
}

button {
    width: 80px; 
    height: 80px;
    border-radius: 50%;
    border: none;
    background-color: hsl(0, 0%, 30%);
    color: white;
    font-size: 2.5rem;
    font-weight: bold;
    cursor: pointer;
    transition: background-color 0.2s, transform 0.1s;
}

@media (max-width: 500px) {
    #calculator {
        width: 95%; 
        max-width: none;
    }

    button {
        width: 100%; 
        height: auto;   
        aspect-ratio: 1/1; 
        font-size: 2rem;
    }
}

button:hover {
    background-color: hsl(0, 0%, 40%);
}

button:active {
    transform: scale(0.95);
}

.operator-btn {
    background-color: hsl(35, 100%, 55%);
}