I've been looking for the same. Now I'll be the first to say I probably didn't do something right following the instruction up here, but I found this and it works perfectly for me.
See this pen by Michael Van Den Berg
#particles-js {
width: 100%;
height: 100%;
background-color: #437993;
/*background-image: url('');*/
background-size: cover;
background-position: 50% 50%;
background-repeat: no-repeat;
position: absolute;
}
canvas {
display: block;
vertical-align: bottom;
}
@*Content you want to be in front goes here*@