I'm not having this issue anymore; I fixed it. But I wanted to post here because it was very specifically a custom e-newsletter theme issue and others are bound to run into it. The theme I was using was based on one of the originals from 4 years ago, when the plugin was still fairly new and going through a lot of changes.
What happened when the 'bad' e-newsletter theme was installed, was a lot of errors running down the screen when I tried to create a new or edit an old theme. I didn't save the error messages. (I was going to submit a support ticket, but decided to try to fix the problem first.)