The preferences page on Test Wikipedia has no tabs and doesn't have the logo overlay: https://test.wikipedia.org/wiki/Special:Preferences
The issue is also visible on the beta cluster: https://en.wikipedia.beta.wmflabs.org/wiki/Special:Preferences
Can't reproduce locally. Seems likely there is some JS/CSS module not loading, although I don't get any JS errors in the console.
Other visibly affected stylesheets:
- oojs-ui dialogs are displaying below the personal bar (T167042)
- mediawiki.notification popups are ugly
- jquery.tipsy popups have larger font size than they should
- jquery.ui buttons are even uglier than usual