I did some more testing and it looks like there is no default skin set. There were some sites I never specifically set DNN-Blue as a skin and it would just work as its skin. Now those sites throw errors. I went through on the host admin and manually changed the skin and the sites load, but this is definitely now how the behaviour should be. How do we fix this?

That's what I have had to do as well, it's a pain for sites that I set up a custom skin for the main site, but left DNN_Blue as the default. Now when my designer logs into an admin that should have the default he gets an error until I can reset the look manually. Any ideas from anyone on how to fix this? I tried doing a search through the source and this method doesn't exist anywhere. Not even sure where it's getting referenced from.

I found how to fix the issue www.dotnetnuke.com/Community/Forums/tabid/795/forumid/107/threadid/182301/scope/posts/threadpage/2/Default.aspx

I don't know why this file wasn't in the upgrade folder, but this totally fixed my issue