changing the layout of the theme

I have the Oblique theme on my web page and use pagebuilder to make pages. However I have some trouble understanding how a theme works relative to a pre-built layout in pagebuilder. If I build a page using pagebuilder the page lays itself on top of the theme-page. The Oblique theme page always has a white square in the middle of every page with a slanted top and bottom edge. Is there any way to remove this white space so that I have just a black background for my page/page layouts? I have not been able to find out how to alter this "start"-page in the Oblique theme. Or is there an easier way to do what Im trying to do: to use pre-built page layouts? I realize that the theme Im asking for help with is the Oblique theme and not the WPMU DEV but im not getting it as an option..