Yes, it its a dark variation of the default XenForo theme available with version 2.3. Users can choose light, dark or system (which uses whatever the user has chosen for their OS system settings.
EDIT:
Well, here we are with our first
Have you seen thread for XenForo 2.3.
It's been a long time coming, and for various reasons that we will go into later, it has changed quite a bit from what we had originally intended it to be, but it's finally ready to start showing you what's new.
tl;dr: there's a video at the end of this post.
Dark mode
A lot of the new stuff in XenForo 2.3 is related to performance improvements that won't be immediately apparent on first glance, but there is one new feature in particular that is very obvious, and that is the new dark mode...