I think something like this should work (you can change the color to be what you want)

.page-PageName .page__main {
  background-color: tomato;
}

Is this just to change the color? I would like to insert an image

Oh, sorry, I misunderstood your question. That would've just changed the color, sorry.

On the example you showed, this is the code they used in MediaWiki:Common.css

.page-Ryuuko_Matoi,
[class*=rootpage-Ryuuko_Matoi] .fandom-community-header__background {
    background-image: url('https://static.wikia.nocookie.net/all-fiction-battles/images/a/a5/Ryuko.jpeg/revision/latest?cb=20230220014432') !important;
    background-size: cover !important;
}

Thanks!

No problem :)