Hi Just,
I am still not sure what your question is, but I can interpret your questions in one of two ways.
1) You want to preview an EmEditor theme before you download.
Well, I use WordPress, and currently I don’t think I can spend time to improve the user interface. I rather want to focus on EmEditor development.
2) You want to preview a theme before you select one from EmEditor configuration properties.
This is already possible. When you select a preview, the selected theme will reflect to the EmEditor window automatically.
I hope this helps.