style.css

html, body, main {
    height: 100%;
    margin: 0;
}

body {
    font-family: "Roboto", sans-serif;
    background-color: #efefefda;
}

main {
    display: flex;
    align-items: center;
    justify-content: center;
}

.calculator {
    box-sizing: border-box;
    border-radius: 15px;
    background-color: rgba(0, 0, 0, 0.911);
    width: 400px;
    padding: 20px;
}

.display {
    text-align: right;
    background-color: white;
    border-radius: 10px;
    color: black;
    padding: 15px 15px;
    margin-bottom: 20px;
}

#top-display {
    font-size: 16px;
    min-height: 20px;
    margin-bottom: 15px;
    color: #3b3737da;
}

#bottom-display {
    font-size: 24px;
    min-height: 30px;
}

.buttons {
    display: grid;
    grid-gap: 10px;
    grid-template-columns:repeat(4, 1fr);
}

.btn {
    padding: 20px;
    border-radius: 5px;
    border: none;
    font-size: 24px;
    background-color: #efefefe8;
    outline: none;
}

.numbers {
    background-color: #efefefbb;
}

.btn:hover {
    background-image: linear-gradient(rgba(0, 0, 0, 0.2) 0 0);
}

.btn:active {
    background-image: linear-gradient(rgba(0, 0, 0, 0.4) 0 0);
}

.first-row {
    grid-column: span 2;
}

#equals {
    background-color: lightgreen;
}

#delete {
    background-color: rgb(202, 66, 66);
}