If the page is short on content, the background doesnt apply over the whole page, and shows the background colour from the html tag. 
 
To reproduce this on phpbb.com, go to your profile page, add an existing site image as background image via inspect element, and delete "<form method="post"..... " to shorten the page content. (or check the behaviour on my board at https://www.rising-dead.com/forums/lotdregistry.php)
 
To make the image fill the space of the html tag also, min-width:100%; has to be added to the body tag. (html, body{min-height: 100%;} )
 I know prosilver doesnt provide an image background by default, but when you add a background, it should work directly, without having to hunt for tweaks to have it display properly. 
 
Reference: 
https://stackoverflow.com/questions/5721904/make-body-fill-entire-screen/5722111