html, body {
    background-color: darkgray;
    width: 100%;
    height: 100%;
    margin: 0;
    text-align: center;
}

/* ---- reset ---- */
body {
    margin: 0;
    font: normal 75% Arial, Helvetica, sans-serif;
}

canvas {
    display: block;
    vertical-align: bottom;
}

/* ---- particles.js container ---- */
#particles-js {
    position: absolute;
    width: 100%;
    height: 100%;
    background-color: #000027;
    /*background-image: url("");*/
    background-repeat: no-repeat;
    background-size: cover;
    background-position: 50% 50%;
}

.main-container {
    position: absolute;
    display: inline-block;
    top: 50%;
    height: 300px;
    width: 300px;
    left: 50%;
    margin-left: -150px;
    margin-top: -150px;
    background-color: #ededed;
    padding: 20px;
    border-radius: 5px;
}