index.css

*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

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

.calculator{
    width: 300px;
    height: 500px;
    box-shadow: 4px 4px 30px rgb(11, 4, 4);
    border: 1px solid white;
    border-radius: 12px;
    background-color: rgb(6, 6, 27);
    overflow: hidden;
}

form input{
    width: 100%;
    height: 100px;
    border: none;
    border-radius: 12px;
    font-size: 2rem;
    color: beige;
    background: rgb(4, 3, 3);
    text-align: right;
    pointer-events: none;
}

button{
    flex: 0 0 22%;
    margin: 8px 0;
    border: 1px solid rgb(28, 12, 32);
    height: 52px;
    width: 60px;
    font-size: 22px;
    font-weight: 600;
    border-radius: 5px;
    cursor: pointer;
}

.buttons{
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
    padding: 30px;
}

.btn-yellow{
    background-color: rgb(228, 208, 26);
}

.btn-white{
    background-color: aliceblue;
}

.btn-red{
    background-color: rgb(153, 29, 29);
}

.btn-green{
    background-color: green;
}