Recently, I was doing my question sweep and noted late posts was overdue for a review. When I opened it, it looked much different! Has a change been made on SE, or is this an opt-in the arqade SE opted into?
Yep, the redesign is part of a wider overhaul of the Review queues that the Stack Exchange team have been (and continue to) work on. See this post on Meta SE for more information: Visual design changes to the review queues.*
It's not something we had to sign up or 'opt in' for - as per the rollout plan at the top of that post, on Feb 23 the design was shipped to all network sites.
* The question is marked featured which usually means that it would be displayed in the yellow announcements widget on the right of the page, but I guess we have so many active Meta posts on Arqade at the moment that it isn't currently showing up there.