How to customize the rendering of the straight lines in mode line?

Question

Here are two examples of what I mean:

1. The lines are rendered in a different color as the background, so they are clearly seen:

2. The lines are rendered in the same color as the background, so they can not be seen:

So, I would like to know how to tweak that, probably setting a face attribute.
Whether to render it clearly or hide it.

Answer 1

Following @lawlist advise, using :box attribute for mode-line-inactive made it.

The mode lines images shown in the question are from inactive frames.
The second image shows an inactive frame with a mode line that does not render the straight lines.

Using:

(set-face-attribute 'mode-line-inactive nil :box t)

renders the straight line with foreground color:

as it is indicated in Face Attributes section of Emacs Lisp manual.