Are you an admin on the wiki you're trying to do this on?

Yes, in fact. The wiki has CSS and javascript implemented, if that makes anything easier.

Add the following code to MediaWiki:Common.css. Replace Page_name with the name of the page you want the code to be applied to.

.page-Page_name .pi-title {
	color: black;
}
.page-Page_name .pi-caption {
	color: black;
}

All in one rule:

.page-Page_name .portable-infobox :is(.pi-title,.pi-caption) {
  color: black;
}

It has not worked. We have the same system in this wiki that the backrooms wiki has, where we have a CSS template to apply CSS code directly to a page. Can I do something like that?

It works. Post a link please.

MediaWiki:Common.css | Glagglepedia Wiki | Fandom

Here you go.

.page-User_Jarameeeeeeeeey_The_Vault .portable-infobox :is(.pi-title,.pi-caption) {
  color: black;
}

Still doesn't work once I implemented the code.

Clear your cache. There are instructions on how to on the common.css page.