Ah, I knew there was a reason! 
@charlie_spring – have you looked at this theme component?
https://github.com/pacharanero/discourse-wikified-post-background-color
It’s posted in the topic you linked in your original post, and you can find instructions on how to install it here: How do I install a Theme or Theme Component?
It doesn’t have friendly settings to change colors and the like, but the theme component gets you part of the way there at least!