I lost almost an hour, trying to do this, so thought I'd share, with a post title that was my keyword search for a solution 😁.
Didn't find the solution using a search engine.
What I wanted to do, is display a Reddit post on my Ghost page, with a white background (as my page's background is black).
It appears that the CSS class which I first looked up with the Developer Tools in my browser was
But for some reason, after multiple refresh, while testing solutions, a new CSS class appeared for the same element:
It appears Reddit is using Embedly
So using Embedly's underlying CSS class name is what did the trick for me 😁