Doesn't it risk changing some stuff I don't want changed, like the background image?

"I just did as you said"

No you actually did not.

I said:

"copy the content of https://villains.fandom.com/wiki/MediaWiki:MP.css to your wiki's MediaWiki:Common.css."

You have to copy the content of MP.css. That's the styles for the affiliate template.

Remove all you pasted there again and copy/paste the content of the page I linked instead.

I did. It worked. But now, how do I restore the Wiki's heading style? It worked when I copied the common,css code for the Wiki, but now it has disappeared.

Nevermind, I forgot to purge the cache of the page. Thank you for you advice. I'm very much a rookie in this admin thing atm

The styles for the heading template were in the other styles, but you have figured this.

However, you missed this snippet which affects light mode. Add it as well.

.theme-fandomdesktop-light .MainPageHeading {
    color: #FFFFFF;
}

Also, you have to import the font used for the heading by adding this line on top of Common.css: (it MUST be on the very top)

@import url("https://fonts.googleapis.com/css2?display=swap&family=Libre+Baskerville");

Where do I put it in the code?

The import has to be on top before anything else, as said.

The other thing you can put anywhere, but best would be at the end since it belongs to the heading template.