style.css

@import url('https://fonts.googleapis.com/css2?family=Press+Start+2P&display=swap');

body {
    font-family: Roboto, sans-serif;
    font-size: 30px;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    gap: 100px;
    margin: 0;
    min-height: 100vh;
}

.header {
    font-family: 'Press Start 2P', Roboto;
    font-size: 50px;
    text-align: center;
}

.app-container {
    display: flex;
    flex-direction: column;
    align-items: center;
    align-self: center;
    border: 6px solid black;
    border-bottom: 15px solid black;
    border-right: 10px solid black;
    border-radius: 8px;
    padding: 50px;
    gap: 20px;
    flex-basis: 600px;
    width: 450px;
    box-sizing: border-box;
}

.display-container {
    font-family: 'Press Start 2P', Roboto;
    display: flex;
    justify-content: flex-end;
    flex-grow: 0;
    border: 3.5px solid black;
    border-bottom: 8px solid black;
    border-radius: 6px;
    min-height: 50px;
    min-width: 340px;
    padding: 5px;
}

.btn-container {
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    min-height: 70%;
    width: 100%;
}

.row {
    display: flex;
    justify-content: space-between;
}

.number-btn,
.operator-btn,
.app-btn {
    font-family: Roboto, sans-serif;
    font-size: 22px;
    font-weight: 900;
    background-color: transparent;
    border: 3.5px solid black;
    border-bottom: 8px solid black;
    border-right: 5px solid black;
    transition-duration: 0.45s;
    min-height: 60px;
    width: 65px;
}

.number-btn:hover,
.operator-btn:hover,
.app-btn:hover {
    background: black;
    color: whitesmoke;
    cursor: pointer;
}

.number-btn:active,
.operator-btn:active,
.app-btn:active {
    background: black;
    color: whitesmoke;
    cursor: pointer;
    border: 1px solid black;
}

.footer {
    min-height: 55px;
    background: black;
    font-weight: 900;
    font-size: 30px;
    color: whitesmoke;
    display: flex;
    justify-content: center;
    align-items: center;
}