You can copy them to your wiki's MediaWiki:Common.css, but you have to be an admin to do this.

Alright. I've taken a break from my role as admin in that Wiki, but I'll try it on the brazilian Heroes Wiki, which I'm currently an admin at.

How exactly do I do this? The Wiki's Common.css page is empty. If I just copy and paste its equivalent from the Heroes Wiki, I'll be also be pasting the names of the staff of that Website, and we don't have a full one yet.

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

Alternatively, you can create MediaWiki:MP.css and then import this page into MediaWiki:Common.css.

If you need further help, please link the wiki you're working on now.

Yes, thanks.

Do what I said above.

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.