body {
    width: 100vw;
    height: 100vh;
    margin: 0;
}

#mainContainer {
    width: 94vw;
    height: 94vh;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

#mainContainer > div:first-of-type {
    display: inline !important;
}