.background {
    height: 100%;
    background: linear-gradient(#98b9d6, #3274ae);
}

.rectangle {
    position:fixed;
    top: 30%;
    left: 50%;
    width:35em;
    height:6em;
    margin-top: -3em; /*set to a negative number 1/2 of your height*/
    margin-left: -17.5em; /*set to a negative number 1/2 of your width*/
    background-color: #3274ae;
    border: 2px solid white;
    text-align: center;
    font-size: 14px;
    color: white;
    font-weight: bold;
}