Get plugin to background of page

Your CSS file is loading fine, yes.

But you need to add some more to it.

z-index is used to say where to place an element in the ‘layers’, when elements overlap.

The higher the z-index the higher it will appear.

add z-index:10 to the header:

header.site-header{
    position: fixed;
    top: 0;
    margin: 0px;
    width: 100%;
    background-color: rgba(0,0,0, 0.6);
    color: white;
    z-index: 10; /* New style */
}

In your case, a z-index of 1 will do it, though.