Board Thread:Support Requests - Designing Your Wikia/@comment-4356266-20160424150338/@comment-24473195-20160424164205

From an analysis of the wiki's history,  it apparently didn't have any portable infoboxes until recently. That seems to be the criteria for the automatic "Europa theme" as nothing changed in older wikis. Also, according to recent changes, the wiki was pretty much "dead" for a couple of months, so that was yet another reason.

>how to get rid of it

You can get rid of it by overriding it with another theme. See Help:Infoboxes/CSS, although that will only work if an admin has another one already defined in special:CSS.