I've made a theme for my forum based on the Cracked theme by Crib. There's a small rounded corner (see attached image, maybe a bit hard to see as it's a light grey round corner on white background) that I'm unable to change, though. The corner seemed to be in mainblock.png, but changing those corners (even filling the entire image file with a single color) doesn't change that corner at all. I've pretty much tried changing every image file I've found that seemed relevant, but none of them have affected those specific corners. Anybody know what file these rounded corners are taken from?
If it's not in the image files it may be in the css. Try inspecting the corner (right click -> inspect element) and looking for "border-radius" in that area.