style.css

html {
    height: 100%;
    width: 100%;
}

body {
    height: 100%;
    width: 100%;
    background-color: #eee;
    margin: 0;
    overflow: hidden;
}

#header {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    padding: 10px;
    position: absolute;
    text-align: left;
    top: 0;
    width: 60%;
}

#footer {
    bottom: 0;
    padding-bottom: 10px;
    position: absolute;
    text-align: center;
    width: 100%;
}

.opgroup {
    display: inline-block;
}

.operation {
    cursor: pointer;
    margin: -4px;
    max-width: 16vw;
}

.button {
    cursor: pointer;
    margin: -5px 0px;
    max-width: 30vw;
}

#overlay {
    background: black url(img/spinner.svg) center center no-repeat;
    height: 100%;
    opacity: .5;
    position: absolute;
    width: 100%;
    z-index: 100;
}