Empty space below Entry/Sidebar

Hi there. I’ve worked off the classic theme to build my own, and am having some trouble with one last bit. I haven’t made a post yet, so the “tallest” item b/w the the header and footer is the sidebar (which I brought under the header/above the footer, on the right still). The space left b/w the footer and what is still just the test entry shows as the page background image. But I’d like it to be white – despite the fact that there’s nothing there. When there are more posts, so that the content area eventually out-lengthen the sidebar, I’d also like there to be white b/w the sidebar and the footer.

Any help? I can copy code here if you like, just let me know. Thanks!