You did a number on that background 🙂
First you have rightly set the background globally in Appearance > Background, which makes it the overall background. That is good
But then, you have also added that same background to the wrapper area, to the header area and to the Content area.
You need to remove the background image from these three areas, AND set their respective background color to transparent
Then you will be left with a single background to remove in the page, which can be done by adding the CSS rule below to the theme Global custom CSS rule box, NOT the per page rule box
body.custom-background.page-id-142 {background-image:none;}