Integration w/ OTP Verification plugin, Button in Register Form is not readable

I am integrating UM with OTP (One Time Password) Verification plugin to obtain verification codes via SMS to complete the member registration. I have correctly configured the form where the integration is carried out, in which an error is generated that prevents it from being displayed correctly.

This is a screenshot of the configuration of the Button to request the OTP Code:???

https://drive.google.com/file/d/1B6WpNjTU7thqEiqbxfxJEuQdmdmwff9f/view?usp=share_link

This is a screenshot of the register page with Verification Button from OTP Verification PlugIn, where you can see that the text color of the generated Button is almost the same as the Button, so it is not readable:???

https://drive.google.com/file/d/16Cke72GAtAZeNWimq6VzN6TP43p73jaA/view?usp=share_link

I have no CSS or other knowledge that would allow me to locate and fix the problem from code. Could you tell me how to correct it?

Thanks in advance.

The page I need help with: [log in to see the link]