Trying to change the background color of one specific page

I am trying to change the background color for this one specific page. All of the other pages on the website have a white background, whereas I need this one to be a dark grey. 

This page used to have a custom code that took away the header/navigation at the top and also made it have a dark grey background, but I need the header. I took that code away, making the background white again. I know I need a specific CSS to input into the page settings, however I am unsure how to do so. I tried a couple times after browsing some of the other discussions on here and every time I try the <style> command that other people were using, it reverts back into that old code that took away that header.

The url for this page is https://www.anspachandhobday.com/london-black

Let me know how I could go about fixing this.

Thanks