Board Thread:Support Requests - Getting Started/@comment-758105-20160819195043/@comment-26437440-20160819195557

In your user preferences you can set your default layout to Monobook, this will make it so that you see Monobook on every wiki you visit. You can also customize the skin on any wiki you create through Monobook.css.

Oasis is the default view now however, and I think there's only a couple of wikis that have been allowed to keep Monobook as their default skin, if that's what you're referring to.