styles.css

.calculator {
    height: 600px;
    width: 400px;
    background-color: black;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    box-shadow: 0 0 10px 5px rgba(0, 0, 0, 0.5);
}

html, body {
    height: 100%;
    margin: 0;
    padding: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
}

.saved-item {
    color: white;
    font-size: 1.5em;
    padding: 10px;
    border-bottom: 1px dotted white;
    text-align: left;
}

#saved {
    color: white;
    position: fixed;
    background-color: rgba(0, 0, 0, 0.5);
    width: 400px;
    height: 600px;
    display: none;
    flex-direction: column;
    text-align: center;
}

#close {
    position: fixed;
    border-radius: 5px;
    width: 24px;
    height: 24px;
    background-color: grey;
    color: white;
    margin-top: 10px;
    margin-left: 165px;
    font-size: 16px;
}

.display {
    padding-top: 10px;
    text-align: center;
}

#display {
    height: 60px;
    width: 95%;
    font-size: 2em;
    color: black;
}

button {
    background-color: green;
    color: white;
    border: black 1px solid;
    width: 77px;
    height: 77px;
    cursor: pointer;
    font-size: 24px;
    border-radius: 5px;
}

.grid {
    display: grid;
    grid-template-columns: 77px 239.77px 77px;
}

button:hover, button:focus {
    background-color: #090;
}

button:active {
    background-color: #070;
}

.buttons {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
}

.number-buttons {
    width: 240px;
    text-align: center;
}

.operator-buttons {
    width: 77px;
    text-align: right;
}

.right-align {
    text-align: right;
}

.stacked {
    width: 77px;
}