Hello there,
This is usually handled by the theme you are using, but yes, it can be changed.
You need to install Simple Custom CSS first and then go to your Dashboard -> Appearance -> Custom CSS.
In there, you need to add the following code:
#pms_register-form input[type="submit"] {
background-color:#fff;
}
#pms_register-form input[type="submit"]:hover {
background-color:red;
}
Just change the colors to something you like, the first color (#fff) is what users see when the page is loaded, while the second one (red) is the color users will see when they place their mouse over the button.
Best Regards.