Hello Peter,
Thanks for updating the thread. 
I checked the website and something which is quite Odd is that I don’t see body information that contains body class in HTML section. That’s the reason why code shared by my colleague @mldarshana is not working. There’s no issue with the code, it’s just the body class information is not present in HTML output, the class .single-post is not taking effect.
I also see that you have enabled Cache plugin. Please deactivate the same temporarily. If you have installed plugin or made any customization in child theme for this specific purpose, please remove that.
Here’s a screenshot that has all the necessary body class information.
Here’s screenshot for your website. As you can see there’s no class information for body.

Thanks.