style.css

body {
    text-align: center;
}

.page > * {
    width: 100%;
    min-width: 350px;
    margin-left: auto;
    margin-right: auto;
    background: goldenrod;
}

@font-face {
    src:url("digital-7/digital-7.ttf");
    font-family: digital-7;
}

.flex {
    width: 300px;
    padding: 30px;
    border: solid 0px white;
    border-radius: 5px;
    display: flex;
    justify-content: space-around;
    margin-bottom: 20px;
}

h1 {
    margin-top: 0;
    margin-bottom: 0;
    text-align: center;
}

button {
    display: inline;
    padding: 5px 10px;
    margin-right: 5px;
    border: solid grey 0px;
    border-radius: 5px;
    font-family: digital-7;
    font-weight: bold;
    letter-spacing: 2px;
    font-size: 15px;
    cursor: pointer;
}

button:hover {
    background: red;
}

.set-zero {
    padding: 10px 20px;
}

.sum {
    background: black;
    font-family: digital-7;
    color: red;
    font-weight: bold;
    font-size: 40px;
    text-align: center;
    padding: 5px 10px;
    margin-right: 5px;
}