Hey @martincons,
Thank you for reaching out! From the looks of it, your active theme is not styling form elements. To style the form, please follow our documentation article on the subject, here: https://slicewp.com/docs/affiliate-forms-not-styled/
If you have any additional questions about styling issues, please open a technical support ticket on our website, here: https://slicewp.com/contact/
Thank you and best wishes,
Mihai