A few tips on creating your own Template Sets.
1. Don't start a Template Set from scratch
The Smarty content used for a few of the pages (like the Form Page and Review Pages) are extremely complex and difficult to reproduce from scratch. I've found that it's far easier to start a new Template Set by basing it on an existing one. There's less work involved and less likelihood of error.
2. Get your design working in your own IDE first
When developing the initial styles and general structure of the pages, I found that scraping the page of an existing Template Set, then modifying it independently of Form Tools and the Form Builder a far more efficacious way to design. Making changes within the Form Builder interface is fine for smaller things, but it's hardly your own, familiar IDE. I found that working this way, then pasting in the CSS and markup back into the Form Builder was a pretty good way to work.