My background

[Felix2]

Add to css_additional.css

/* Start my background */
body {
    background: url("/images/space-bg.jpg") no-repeat center fixed;
    background-size: cover;
}

html {
  background-image: url('/images/space-bg.jpg');
}

#content {
    background-color: rgba(20,20,20,0.8); //white with 80% transparency (adjust as needed)
}
/* End my background *

Where IMAGES is the folder and space-bg.jpg is the background image you want to use.