Control the two vertical strips on both sides of the Emacs frame was always a problem to me, until yesterday.
The annoy part of this problem is when you have a dark-background theme, you don’t want to have the two white bright bands on both sides. I found yesterday that "fringe" is the name of the face.
So you can customize-face it, and make it something like:
(custom-set-faces ‘(fringe ((t (:background "grey10")))))