Is there a way to have a custom theme built?

I understand that you want to build your custom theme for your JotForm. If so, you may build your theme from the Advanced Form Designer for your form. Please take a look at the following guide that is provided with basic videos to help you with using Form Designer.

https://www.jotform.com/help/300-Form-Designer-Tutorial-Let-s-create-fantastic-forms-

We have hundreds of themes already available in our theme-store that you may be interested to take a look.

Hope this information helps!