<link rel="stylesheet" type="text/css" href="css.php?css=xenforo,form,public&style=1&" />
And there go all additional css templates, for e.x in threads we have:
<link rel="stylesheet" type="text/css" href="css.php?css=attachment_editor,bb_code,editor_ui,message,message_user_info,quick_reply,share_page,thread_view&style=1&" />
The problem is that if we want to modify some design that is set in second css.php output we have to use !important, which will overwrite any css that we've set for some class, id. But if we include extra.css in the second output we don't have to do this anymore. I think it is more a bug because extra.css functionality is seriously limited as it works now.