html, body {

    height: 100%;
}

body {
    background-color: dimgray;
    margin: 0;
    padding: 0;
    display: flex;
    flex-direction: column;
}

.mainbox {
    background-color: azure;
    padding: 5%;
    border: 10%;
    border-radius: 100px;
    flex-grow: 1;
    margin: 5%;
    
    text-align: center;
    
}