“Content Sidebar” Problem and Cannot Remove “Primary Sidebar” and “Secondary”

Good day!
This is about editing the sidebars in a particular page.
I have been trying to place a full width sidebar (with background image) at the “Content Sidebar” area of the template, but there always appears white spaces at both the left and right side so that the background image does not appear completely. What is the problem here? And how will I resolve it?

I was thinking that perhaps it was because there are sidebars placed at the “Primary Sidebar” and “Secondary” area of the template which occupy both sides of “Content Sidebar”. The problem is whenever I try change the value of “Primary Sidebar” and “Secondary” to “[None]” and click on “Update”, the change does not appear. The original values remain in place. Is this really supposed to be like this?

Thanks in advance for all the help you can give.