main.css

body {
    height: 2000px;
}

.box {
    height: 100px;
    width: 100px;
    margin-bottom: 10px;
}

.container {
    position: absolute;
    top: 50%;
    margin-top: -50px;
    width: 100%;
}

.bluebox {
    background-color: blue;
    margin: 0 auto;
}