Custom Corner Radius Controls
The Panels stack has a Round Corners? checkbox setting, but there are no controls for you to determine how round the corners are.
This is because the settings for round corners is actually set in the Site Styles stack settings, under the Site Sizing settings.
So if you want to adjust the roundness of the corners on your Panels stacks, you need to do it in the Site Styles stack.
Now go forth and make your websites great!