If you have anything you need to make rounded inside of reports, you can use Containers as they have the ability to have rounded corners.
In this article we will walk through editing a container's corners to make custom measurements, and also set a default.
When you go into the properties area for the container under boarder you will see Rounding Radius.
Click on it to customize it, or you can enter a number there out of your unit for the designer. You can also set the color.

Now you will drag in a Table with transparent header and footer inside a container for an example. As seen below, the table pulls the background color to show this is working as expected.

When this report is ran you will see the curves are cut out of where the table overflowed in the designer. So make sure you do not accidentally removing content with a corner.

Here is the doc page for containers:
https://developer.mescius.com/activereportsjs/docs/ReportAuthorGuide/Report-Items/Supplemental-Report-Items/Container#general-container-configuration
Victor Stahlman