Extension:AjaxLogin

AjaxLogin extension allows users to log in through an AJAX pop-up box. Instead of being taken to Special:UserLogin when they click "Log in / Register", an AJAX pop-up box will be presented to them.

Parameters
The  controls whether AjaxLogin is enabled for the current skin or not. By default, AjaxLogin is enabled for all core MediaWiki skins (Cologne Blue, Modern, Monobook and Vector).

If you have multiple custom skins for which you'd want to enable AjaxLogin, you could add something like this to your wiki's, after the extension's  :