Sounds like a theme styling issue, as the signup form doesn’t have any of it’s own CSS styles, it’s just standard form fields, with each field inside its own paragraph tab. Each form field uses a simple label field followed by the input field. All very common and straight ahead.
The whole signup form is wrapped in its own div, to make it easy to target those specific form fields with CSS in your child theme to override whatever messed up CSS styles your theme is applying to form fields.
Use your browser’s inspect element feature to view the CSS being applied to each form element, and then use that info to create your own CSS to change what your theme is doing.