style.css

* {
    padding: 0;
    margin: 0;
}

button {
    border: none;
    transition: 0.5s;
}

.calculator {
    display: flex;
    flex-flow: column;
    flex: 0 0 auto;
    height: 100vh;
    justify-content: center;
    align-items: center;
    overflow: hidden;
}

.row {
    display: flex;
    width: 360px;
    height: 90px;
}

.calculator,
.row {
    gap: 2px;
}

.col {
    border-radius: 15px;
    flex: 1;
    font-family: "Ubuntu Mono", monospace, sans-serif;
    font-size: 1.8rem;
    font-weight: 600;
}

.col.digit.zero {
    flex: 2;
}

.row.input {
    align-items: center;
    text-align: right;
}

.row.input .col {
    height: 100%;
    display: flex;
    justify-content: flex-end;
    align-items: center;
    padding: 0 20px;
    background-color: midnightblue;
    color: azure;
}

.digit {
    background-color: rgb(12, 12, 51);
    color: azure;
}

.digit:hover {
    color: midnightblue;
}

.operator {
    background-color: burlywood;
    color: rgb(12, 12, 51);
}

.misc {
    background-color: chocolate;
    color: lemonchiffon;
}

.digit:hover,
.misc:hover {
    background-color: bisque;
    color: midnightblue;
}

.operator:hover {
    background-color: midnightblue;
    color: bisque;
}