It would be unusual for Formidable not to work with a theme. Formidable is a WordPress plugin. As such, it is designed to be compatible across the entire WordPress theming spectrum. I've seen it work with Oxygen which deactivates the WordPress theming interface. Formidable has its own CSS styles that you can choose to use or override with your own.

As an FYI, there are literally tens of thousands of WordPress themes on the market. It's impossible for any plugin development team to test its product against every single theme. If you find that you have issues with your theme, open a support ticket with Formidable. The Formidable support team is very good at solving problems.