Seen on https://nostalgia.wikipedia.org/wiki/Wikipedia_utilities but applies on other pages as well.
The go button for the special page selector is styled to look like an OOUI button. This looks really weird on a website that is supposed to be frozen in time.
The button is styled because $wgUseMediaWikiUIEverywhere is true, which adds the css classes mw-ui-button mw-ui-progressive to the button in [[ https://phabricator.wikimedia.org/source/mediawiki/browse/master/includes/Xml.php$459 | Xml::submitButton ]]. Nostalgia calls this function from [[ https://phabricator.wikimedia.org/diffusion/SNOS/browse/master/NostalgiaTemplate.php$356 | NostalgiaTemplate ]].
A quick hack could be that $wgUseMediaWikiUIEverywhere will be set to false, but a better approach would probably be to replace the method call and manually construct the button.