Nightly? Rated 4 out of 5 stars

I love this theme, or at least the one that gets placed. But I see the different ones (Nightly mainly is the one I want). How can I change the skins? I can't figure this out. Please help!

This review is for a previous version of the add-on (4.6). 

Version 4.6.3
https://addons.mozilla.org/en-us/firefox/addon/mx3/versions/