I see there is an admin.master and admin1.master. I am not sure if there is a reason to have both but I did find that they both use the same name for the code-behind class which was causing a problem. Also, admin1.master was referencing admin.master.cs
instead of admin1.master.cs as the code-behind which added to the problem beyond the class name conflict. Once I fixed the reference to the code-behind and added 1 to the end of the class name everything compiled without a problem.
I also added a prefix to each of the classes for the Site.master in each theme. They all use the same class name which causes a conflict when merging the assembly when using a Web Deployment Project. I also had to add a prefix for a couple of the Sidebar user
controls in the themes that were conflicting.
I could package up my changes and send them if you are interested in using these changes.