Board Thread:Support Requests - Designing Your Wiki/@comment-38796084-20191125151544/@comment-23989026-20191126125736

The help page says that everything goes through MediaWiki:Wikia.css

Placing .portable-infobox and other PI-specific CSS in a separate stylesheet (MediaWiki:Themes.css) imported to MediaWiki:Wikia.css can also be helpful. The import line in the main skin stylesheet @import url("/load.php?mode=articles&articles=MediaWiki:Themes.css&only=styles"); will load the Themes.css in an optimized manner.

An example on w:c:supersmashbros:MediaWiki:Wikia.css