style.css

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

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

.container {
    background-color: black;
    height: 840px;
    width: 500px;
    border-radius: 50px;
    padding: 1.5rem;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    grid-row: repeat(5, 1fr);
    grid-gap: 10px;
    margin-top: 250px;
}

.button {
    color: white;
    width: 100px;
    height: 100px;
    background-color: #1A1A1A;
    border-radius: 50%;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 2.8rem;
    cursor: pointer;
}

.button:hover {
    opacity: .5;
}

.function {
    background-color: #A5A5A5;
    color: black;
}

.operator {
    background-color: #F1A33C;
}

.number-0 {
    grid-column: 1 / span 2;
    width: 215px;
    border-radius: 60px;
    justify-content: flex-start;
    padding-left: 39px;
}

@media screen and (max-width:1400px) {
    .container {
        height: 640px;
        width: 350px;
    }

    .buttons {
        margin-top: 220px;
    }

    .button {      
        width: 65px;
        height: 65px;
        font-size: 2rem;      
    }

    .number-0 {
        width: 136px;
        padding-left: 21px;
    }   
}

@media screen and (max-width:500px) {
    .container {
        height: 430px;
        width: 240px;
        border-radius: 40px;
        padding: 1rem;
    }

    .buttons {
        margin-top: 150px;
    }

    .button {      
        width: 42px;
        height: 42px;
        font-size: 1.3rem;      
    }

    .number-0 {
        width: 95px;
        padding-left: 16px;
    }   
}