How can I give a Combo Box's context menu a bevel outline?

Hi there,

I’m wondering how to give the context menu that appears when clicking on a Combo Box a rounded outline, but don’t know what value to look for. The style menu is like an entire library. And yes, I’ve tried the Border Brush value in the Combo Box Style sub-category, but that doesn’t seem to do it. (See below)

image
image

Any help is appreciated.
Thank you!