Thanks for your question about changing the background for a single page instead of the entire website. The pages or posts backgrounds can be changed per section or block. You have to edit the page and then select the section where you want to change the background. If you want the block to encompass the whole page, then you can make the block do so by simply dragging down to expand it. You can see in the screenshot below that you simply need to click on the toolbar to change the image/color/pattern of the section background.