Perspective table design

You can find settings like this using the browser Developer Tools. (F12 on most browsers.)

I was able to turn it off using Styles | Advanced Stylesheet (available at least on 8.1.25) and adding the following CSS.

.ia_table__headContainer, .ia_table__footContainer {
	box-shadow: none;
}

This will affect all tables in the project.
There's probably a way of injecting this into one table but I haven't time to figure it out.