Are we still on beta? Most sites of the network have a custom design, which they got when they were out of beta.
Graduation and design have been made independent. We graduated in summer 2016. As far as I know, there is no public design queue. But, you could have a look at all sites ordered by traffic. If I were SE, I would give high-traffic sites higher priority to get a custom design. And, sadly, there are still several higher-traffic sites without custom design. So, we'll probably just have to wait a bit.