Changing individual blog page on 7.1

I run a website with a significant visual element, and I am attempting to change the colour of an individual blog post. I found CSS solutions for people having this issue in 7.0, but haven't found anything for 7.1. 

I also tried inserting custom CSS that @tuanphan had posted elsewhere, but it did not work for me in this case. I am a CSS newbie, so it could be simply that I had issues finding the correct section-id when I inspected the elements on the page.

Your help would be so very appreciated!

The website is https://www.oursingingbodies.com/the-project and I would like the background of posts to be white but the most recent post (not yet published) to have a black background. 

White Background of post items, like this

or background of entire /the-project page?

Hi @tuanphan! Background of the entire page! I want to keep this one black, because it works a lot better aesthetically -- https://www.oursingingbodies.com/the-project/y022ie9c9qnduzg3a3y0afah5q76w6

but the rest of the blog white. 

@tuanphan Would you mind showing me how you managed to change the entire page background to white? Thank you SO much for your help, it's so appreciated.

Thank you @tuanphan! You're amazing!