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 reddit-post.

But for some reason, after multiple refresh, while testing solutions, a new CSS class appeared for the same element: embedly-card.

It appears Reddit is using Embedly

Embedly makes your content more engaging and easier to share | Embedly
Embedly delivers the ultra-fast, easy to use products and tools for richer sites and apps.

So using Embedly's underlying CSS class name is what did the trick for me 😁

.embedly-card {
background: #FFFFFF
}