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.

It works. Clear your browser cache.

It works. Thanks!