Hello,

1) For the button color try adding the following Custom CSS in the Global Custom CSS area under Theme Settings >> Custom CSS.

a.btn.wd-switch-to-register {
    color: white;
}

2) The text is coming from Theme Settings >> Login/Register. You can change it from there.

Screenshot for Clarification: https://gyazo.com/d1f7b55809b3687598f35509c79f15be

Best Regards