PS: If you haven’t resolved this it is because the Kubrick theme uses two background images.
One image is mostly white with a gray area where the sidebare will be. This is used on the main page.
The second image is completely white.
You’ll need to replace the background image on single.php with the same background image from the main block.
PPS: Find a custom theme, and modify it. Kubrick theme doesn’t take well to modifications. I was pulling my hair out, then I switched and it has been smooth sailing since.