The gallery integration into PowerStore was meant for the specific type of gallery being added to the description of the product, not on an entirely newly created page.
First thing you'll have to do is to go to change the background on the contentWrapper. This can be done in the common.css file or directly on the Gallery.php page. If you make changes in the common.css file, it will affect other pages as well. You would want to find the css block that looks like this in the common.css file:
You will also have to change the height of the contentBox div. You can do this in the content.css file or directly on the page. In the content.css file, you would look at this css block:
Again, if you change it in the css file, it will affect other pages as well.