ok I’ve found the reason: you have the XWiki Repository Application extension installed in your wiki and this extension overwrites some CSS styles (see xwiki-platform/xwiki-platform-core/xwiki-platform-repository/xwiki-platform-repository-server-ui/src/main/resources/ExtensionCode/ExtensionSheet.xml at 8119b9201ff00c1eb26f7f3d34f078c720c97fea · xwiki/xwiki-platform · GitHub).
Would be great if you could report a bug at XWiki Platform - XWiki.org JIRA
To fix this, you could either uninstall this extension if you don’t use it or try editing the ExtensionCode.ExtensionSheet page using the object editor and change the StyleSheetExtension xobject to use “on demand” instead of “always” (I’ve not tried it, it could have some side effects on the Repository app’s styling).
Thanks