empress-blog currently only supports 2 themes, the default Casper template and the Attila template. If you want to swap between templates it is as simple as installing a different npm dependency 🎉
The empress-blog documentation recommends that you use the default theme called Casper, which is designed to be a clean, readable publication layout and can be easily adapted for most purposes. The empress-blog-casper-template is a direct fork of Ghost's official Casper repository so it aims to be as close as possible to the original.
If you are interested in writing your own theme for empress-blog you can get started using
npm init empress-blog-template <template-name>
and it will create a brand new template addon in the folder empress-blog-template-name-template! You can check out the documentation for create-empress-blog-template here. As Ghost uses handlebars it is not too difficult to port an existing Ghost template to use Ember templates, so if you have any requests for an existing open source Ghost template please let us know 👍
If you do end up writing an empress-blog template please let me know on Twitter so I can include your template in a list of existing templates.