Adding background to a page in 7.0, but normal codes not working

Hello everyone!

Thank you so much in advance for anyone who can provide help. I am looking to change the background color of one page of my website, but all the current codes do not seem to work. I'm a coding novice, but not a complete novice, and have done some HTML and CSS before on my sites -- so, I'm pretty adapt usually at finding the right codes from all the wonderful people to help here!

However, I cannot seem to crack this puzzle for a really important page of my site. I am attempting to create a semi-standalone page -- beyond just a cover page -- for a section of my website, and I've already removed the header/navigation and footer through code. Yet, I am unable to change my background of the main part of the website.

Here is the website I am trying to change (it's obviously in its earliest phases): https://www.barrygoldenberg.com/harlemprepbook

Please let me know if you have any suggestions. I'd like to change the entire background. I am using the Pacific template in 7.0.

Thank you so much!

Hi tuanphan -- oh my! Thank you *so* much! This worked perfectly! That makes sense and I truly appreciate this so much! Thank you so much for your quick help. I am adjusting my background now! This is very much appreciated.