How to add a border to the commander plugin's mobile toolbar

Hi, I need some advice.
The problem: In the mobile version of the commander plugin the mobile toolbar above the keyboard of the same color as the background of the page, and I don’t like how it kida gets lost.
I would like to add a simple border to separate the two parts. However, being on mobile I don’t know how to inspect the page and therefore I haven’t yet managed to create a css that can do this.

To clarify, here there is an image to visualize what I would like to obtain - a top border with the same color as the main one:

What can I do?