styles.css

html, body {
    background-color: #bdbdbd;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    height: 100%;
}

#calculator-container {
    display: flex;
    flex-direction: column;
    color: white;
}

#calculator-container .row{
    display: flex;
    height: 100px;
}

#calculator-container .row.display {
    justify-content: flex-end;
    font-size: 3em;
    line-height: 2.9;
    padding-right: 10px;
    background-color: #374046;
}

#calculator-container .row .btn {
    padding: 10px;
    width: 100px;
    line-height: 3;
    text-align: center;
    font-size: 1.7em;
    border: 1px solid black;
    background-color: #757575;
}

#calculator-container .row .btn.large {
    flex-grow: 1;
}

#calculator-container .row .btn:hover {
    cursor: pointer;
}

#calculator-container .row .btn.active {
    background-color: #b36b00;
}

#calculator-container .btn.misc {
    background-color: #616161;
}

#calculator-container .btn.operation {
    background-color: #ff9800;
}

#calculator-container .row:last-child .btn:last-child {
    background-color: #ff5722;
}