It actually happens only on farms configured without the "path mode" option.
The issue is that the icons of the top menu are not displayed well, because the glyphicon font URL is relative to the main wiki URL, not the current wiki URL, and for some reason, the browser forbid this cross-site loading.
Steps to reproduce:
- set xwiki.virtual.usepath=0 in xwiki.cfg
- on the main wiki: customize XWiki.DefaultSkin:
- on the field style.css, paste the content of the file style.css that is inside the skins/flamingo folder.
- Go to a subwiki
- Change the skin to: xwiki:XWiki.DefaultSkin
- Refresh your browser cache