Ultimatum drag and drop set up

Hello,

I have Ultimatum Drag and Drop theme installed on a multisite install. I am looking for same direction on how to leave my current website up, while I configure the new theme.

In the past, i simply took the site down over night, but this theme is gonna take a little more effort. How can I work on one theme, leave another in play, then transfer the new theme into play?

Thanks!