How do I change the background image on one page, without it affecting my home page?

Hello There!
I have a relatively simple website. The main thing I care about is getting form submissions. When a customer successfully submits to the form, I have them directed to a success page. The success page has a background image that is left over from the template. I cant figure out how to change that background image without disrupting my home page.
Thank you so much for your time.

Here is my site

You should be able to select the element with the background image, add a combo class, then change the background image on that one instance.