How to delete individual installed themes.

I am trying to set up my first multisite install. Before I realized that this is the way to go for my project, I had already installed some themes to my primary site. Since then, I sat up my mu structure, but now I find that I can't upload or install any new themes or plugins to individual sites in the network. There is simply no link to add new to the list. And, I have no idea how I can delete the existing installed themes in the individual sites.

I am excited about getting this thing up as fast as possible, but I have hit a roadblock. Please help.