Adding a glow overlay

I would like to insert a “glow” (radial gradient that goes from white to transparent) over top of every page on my site. I can make the glow in photoshop, I just need to know how to code it in. I know I need to edit the Header section of the style.css, then add some code to header.php, but I don’t know what. Here’s my attempt (I’m not much of an HTML/CSS coder)

<script>
#glow {
    background-image: url('/image.png');
    position: absolute;
    margin: auto;
}
</script>

<div class = "glow">
    insert something here
</div>

If someone could help, that would be great. Thanks