Derp. I actually just edited the Category Strip style property under Forum / Node List. I couldn't find it earlier because I was looking at the Sub-Heading style property under Building Blocks since that's what controls the background there, so I figured it got the radius, too.
Wanna hear something funny... I'm very artistically challenged (can't make a style myself that looks worth a darn but can modify an exisitng) but the technical questions I can answer.