style.css

* {
    transition: all .1s ease;
}

body {
    /* remove default extra space */
    margin: 0px;
    display: flex;
    align-items: center;
    justify-content: center;
    font-family: 'Courier New', Courier, monospace;
}

.button {
    width: 100px;
    height: 100px;
    margin: 6px;
    font-family: 'Courier New', Courier, monospace;
    border-radius: 100px;
    border-color: darkolivegreen;
    background-color: rgb(39, 52, 17);
    color: white;
}

.button-container {
    max-width: 500px;
    width: 500px;
    max-height: 500px;
    height: 500px;
    display: flex;
    flex-direction: row;
    flex-grow: 1;
    flex-wrap: wrap;
    align-items: center;
    justify-content: center;
}

/* click transition buttons */
button {
    height: 70px;
    width: 150px;
    background-color: #3399FF;
    border: lightblue solid 1px;
    outline: none;
    color: #fff !important;
}

button:active {
    -webkit-box-shadow: inset 1px 1px 10px #333;
    -moz-box-shadow: inset 1px 1px 10px #333;
    box-shadow: inset 1px 1px 10px #333;
}

.display {
    display: flex;
    justify-content: flex-end;
    align-items: center;
    border: 5px inset black;
    max-width: 450px;
    width: 400px;
    max-height: 30px;
    height: 30px;
    padding: 4px;
    margin-bottom: 18px;
    margin-top: 36px;
    font-size: larger;
    background-color: darkolivegreen;
    color: white;
}

.container {
    margin: 20px;
    display: flex;
    flex-direction: column;
    align-items: center;
}

.number-button {
    max-width: 100px;
    width: 100px;
    max-height: 100px;
    height: 100px;
    margin: 6px;
    font-size: 20px;
    font-family: 'Courier New', Courier, monospace;
    font-weight: bold;
    border-radius: 100px;
    border-color: darkolivegreen;
    background-color: rgb(7, 92, 7);
    color: white
}

.operate-button {
    max-width: 100px;
    width: 100px;
    max-height: 100px;
    height: 100px;
    margin: 6px;
    font-size: 20px;
    font-family: 'Courier New', Courier, monospace;
    border-radius: 100px;
    border-color: darkolivegreen;
    background-color: rgb(44, 56, 23);
    color: white;
}

#equal {
    font-weight: 600;
    font-size: 20px;
    font-family: 'Courier New', Courier, monospace;
    border-radius: 100px;
    border-color: darkolivegreen;
    background-color: rgb(16, 20, 9);
    color: white;
}

#clear {
    font-size: large;
}