style.css

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

body {
    font-family: 'Montserrat', sans-serif;
    font-size: 1.2rem;
}

.calculator {
    margin-top: 1rem;
}

.calculator__inner {
    width: 400px;
    height: 540px;
    border: 2px solid black;
    border-radius: 4px;

    margin: 0 auto;
}

.calc_top {
    height: 30%;
    background-color: rgb(20, 19, 19);
    color: rgb(255, 240, 240);
    text-align: center;
}

.calc_top-title {
    height: 45px;
    padding-top: 0.8rem;
    border-bottom: 2px solid white;
    padding-bottom: 3.6rem;
}

.calc_top-output-field {
    margin-top: 1rem;
    padding: 0 0.7rem;
    font-size: 2.7rem;
    text-align: right;
}

.calc_top-output-field-min {
    margin-top: 1.7rem;
    padding: 0 0.7rem;
    font-size: 1.7rem;
    text-align: right;
}

table, td, tr {
    border: 1px solid black;
    text-align: center;
}

tr:first-child button {
    background-color: #dadada;
}

td{
    width: 99px;
    height: 75px;
}

td:last-child button{
    background-color: #e98e31;
}

table {
    border-collapse: collapse;
}

button {
    width: 100%;
    height: 100%;
    padding: 1rem;
    font-size: 2rem;
    font-weight: 400;
    background-color: white;
    text-decoration: none;
    border: none;
}

td:hover button {
    font-weight: 600;
    cursor: pointer;
    opacity: 50%;
}