Forgive my ignorance, but why do sheva brachos need a theme?
And on a related note, I once was one of the hostesses for a sheva brachos with a “theme.” The theme was international. One of the hostesses was a professional chef and wanted to show off.
We had guacamole, fish toast, vegetable soup, kim chee, some kind of chicken, creme-filled pastry swans and “spaghetti & meatballs” cake. There was other stuff, I just can’t remember it.
The vegetable soup was burnt, the kim chee had not marinated nearly long enough, and the “spaghetti & meatballs” cake was just plain stupid. There were a lot of other problems, too.
The only theme you need is good food and people who want to be somayach the chasson and kallah.