Reusable Custom Form Themes

With Nintex for SharePoint on-prem, we're able to create a form theme that we want to re-use and apply it as a default template. This saves a lot of time setting colors, banners, and other theme items. I see that there are built-in themes for NAC, but there's no way to save a custom theme for re-use. Ideally, we could have the ability to set a default template for start forms. That way, we can start designing forms without having to first theme them.