[Theme: Duena]Changing background color for page wrap on a single page

Hello, I’m fairly new to coding and have only done the codecademy course. I’m working on a WP website for a school team and am up against a deadline for when it has to be done,so I don’t have anymore time to try and figure it out myself. https://www.schescience.org/?page_id=35 Here is the page i need to edit. I want to change the page wrapper to be white so it will blend in with the wiki. How would I go about doing this, and also why does .page_wrap.page-id-35{ background-color: #ffffff;} not work? Also what would you recommend for learning more advance design and programming aft codecademy?