Color of Kubrik Sidebar

On my page (www.fmueller.com), I am using the default theme (Kubrik) with some modifications. It appears that the sidebar shows up with a very light gray background – it’s so light that it’s hard to see on some monitors – while the background of the main text is pure white. I can modify the width of the text in the sidebar, but not the gray background, by changing the sidebar width in the stylesheet. I also checked the kubrickbg.jpg and kubrickbgwide.jpg images, and the gray doesn’t come from them. I’d like to change the width of the gray area because it overlaps with content outside of the sidebar, and I’d also make the color a bit more obvious. How can I do this? Many thanks in advance!