It took much, much longer than I anticipated but I finally released the first version of the Designer add-on for WP Terms Popup. This add-on is a replacement for the old Pro add-on which lets uses customize the look of their plugins without having to know CSS or write any code.
Designer is much more flexible and easier to use than the Pro add-on ever was. It was immediately more successful than its predecessor in terms of sales (although that's not really saying much).